Positive electrode active material for secondary battery, secondary battery, battery management unit, and electronic device

ABSTRACT

A lithium-ion secondary battery including a lithium-containing complex phosphate as a positive electrode active material is provided. Furthermore, a positive electrode active material with high diffusion rate of lithium ions is provided to provide a lithium-ion secondary battery with high output. A positive electrode active material of a lithium-ion secondary battery includes a first plate-like component and a second plate-like component, a third prismatic component between the first component and the second component, and a space between the first component and the second component.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a lithium-containing complex phosphate.The present invention also relates to a lithium-ion secondary batteryincluding an electrode in which a lithium-containing oxide is used as anactive material. The present invention also relates to a batterymanagement unit and an electronic device.

2. Description of the Related Art

In recent years, power storage devices such as lithium ion secondarybatteries have been developed.

Examples of such power storage devices include a power storage devicehaving an electrode formed using lithium iron phosphate (LiFePO₄), whichis a composite oxide, as an active material. The power storage devicehaving an electrode formed using LiFePO₄ has high thermal stability andfavorable cycle characteristics.

As an example of a method for forming a composite oxide such as LiFePO₄,a hydrothermal method can be used (e.g., Patent Document 1). Ahydrothermal method is a method, which is performed in the presence ofhot water, for synthesizing a compound or growing crystal of a compound.

By using a hydrothermal method, even a material which is less likely tobe dissolved in water at ordinary temperatures and pressures can bedissolved, and thus a substance which is hardly obtained by a productionmethod performed at ordinary temperatures and pressures can besynthesized or crystal growth of such a substance can be conducted.Further, by using a hydrothermal method, microparticles of singlecrystals of an objective substance can be easily synthesized.

Using a hydrothermal method, for example, enables a desired compound tobe formed in the following manner: a solution containing a raw materialis introduced into a container resistant to pressure and be subjected toheat and pressure treatment; and the treated solution is filtered.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2004-95385

SUMMARY OF THE INVENTION

It is highly demanded that in-vehicle secondary batteries for HEVs, EVs,PHEVs, and the like and stationary secondary batteries among lithium-ionsecondary batteries for a variety of uses have higher output. To achievehigh output of the lithium-ion secondary battery, the electrode reactionspeed and the diffusion rate of lithium are required to be high. Theoutput of the lithium-ion secondary battery can be increased byincreasing the diffusion rate of lithium in a composite oxide such asLiFePO₄ used as a positive electrode active material.

In view of the above, an object of one embodiment of the presentinvention is to provide a composite oxide such as LiFePO₄ with highdiffusion rate of lithium. Another object of one embodiment of thepresent invention is to provide a positive electrode active materialwith high diffusion rate of lithium. Another object of one embodiment ofthe present invention is to provide a lithium-ion secondary battery withhigh output. Another object of one embodiment of the present inventionis to provide a novel battery.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the invention disclosed in this specification is apositive electrode active material of a lithium-ion secondary batteryincluding a first plate-like component, a second plate-like component, athird component between the first component and the second component,and a space between the first component and the second component.

In one embodiment of the present invention, the first component includesa first surface, a second surface, and a third surface. The firstsurface has a larger area than the third surface. The second surface hasa larger area than the third surface.

In one embodiment of the present invention, the first surface and thesecond surface do not share a side.

In one embodiment of the present invention, the first surface has asubstantially n-gon shape (n is a natural number of 5 or more), and atleast one of interior angles of the n-gon shape is 100° or less.

In one embodiment of the present invention, the third component has asubstantially polygonal columnar shape, and one or a plurality of thethird component is included between the first component and the secondcomponent.

In one embodiment of the present invention, the first component, thesecond component, and the third component include lithium (Li), a metal,phosphorus (P), and oxygen (O), and the metal includes at least one ormore elements selected from iron (Fe), manganese (Mn), cobalt (Co), andnickel (Ni).

In one embodiment of the present invention, the first component, thesecond component, and the third component include lithium (Li), iron(Fe), phosphorus (P), and oxygen (O).

In one embodiment of the present invention, the first component, thesecond component, and the third component include lithium ironphosphate.

In one embodiment of the present invention, the first component, thesecond component, and the third component include LiFePO₄.

One embodiment of the present invention is a secondary battery includinga positive electrode including the positive electrode active materialaccording to any one of the above embodiments of the present invention,a negative electrode, and an electrolyte.

One embodiment of the present invention is a battery management unitincluding the secondary battery according to any one of the aboveembodiments of the present invention and a control circuit.

One embodiment of the present invention is an electronic deviceincluding the secondary battery according to any one of the aboveembodiments of the present invention and a power switch.

One embodiment of the present invention is an electronic deviceincluding the secondary battery according to any one of the embodimentsof the present invention and a display device.

One embodiment of the present invention is an electronic deviceincluding the secondary battery according to any one of the embodimentsof the present invention and an input-output terminal and theinput-output terminal is configured to perform wireless communication.

A positive electrode active material with high diffusion rate of lithiumcan be provided. One embodiment of the present invention can provide acomposite oxide such as LiFePO₄ with high speed diffusion rate oflithium. One embodiment of the present invention can provide alithium-ion secondary battery with high output. One embodiment of thepresent invention can provide a novel battery.

Note that one embodiment of the present invention is not limited tothese effects. For example, depending on circumstances or conditions,one embodiment of the present invention might produce another effect.Furthermore, depending on circumstances or conditions, one embodiment ofthe present invention might not produce any of the above effects.

BRIEF DESCRIPTION OF TUE DRAWINGS

In the accompanying drawings:

FIG. 1 shows a method for manufacturing a lithium-containing complexphosphate;

FIGS. 2A1 to 2A6 and 2B1 to 2B6 each show a schematic view of alithium-containing complex phosphate;

FIGS. 3A to 3C illustrate an example of a secondary battery and examplesof electrodes;

FIGS. 4A and 4B illustrate an example of a secondary battery;

FIGS. 5A and 5B illustrate an example of a secondary battery;

FIGS. 6A and 6B illustrate an example of a secondary battery;

FIG. 7 illustrates an example of a secondary battery;

FIGS. 8A and 8B show an example of a method for manufacturing asecondary battery;

FIGS. 9A to 9C show an example of a method for manufacturing a secondarybattery;

FIG. 10 shows an example of a method for manufacturing a secondarybattery;

FIGS. 11A to 11D illustrate a curvature radius;

FIGS. 12A to 12C illustrate a curvature radius;

FIGS. 13A to 13C illustrate a coin-type secondary battery;

FIGS. 14A and 14B illustrate a cylindrical secondary battery;

FIGS. 15A to 15E illustrate flexible laminated secondary batteries;

FIGS. 16A and 16B illustrate an example of a power storage device;

FIGS. 17A1, 17A2, 17B1, and 17B2 illustrate examples of power storagedevices;

FIGS. 18A and 18B illustrate an example of a power storage device;

FIGS. 19A and 19B illustrate an example of a power storage device;

FIG. 20 illustrates an example of a power storage device;

FIGS. 21A and 21B are diagrams of application examples of a powerstorage device;

FIG. 22 is a block diagram illustrating one embodiment of the presentinvention;

FIGS. 23A to 23C are conceptual diagrams illustrating one embodiment ofthe present invention;

FIG. 24 is a circuit diagram illustrating one embodiment of the presentinvention;

FIG. 25 is a circuit diagram illustrating one embodiment of the presentinvention;

FIGS. 26A to 26C are each a conceptual diagram illustrating oneembodiment of the present invention;

FIG. 27 is a block diagram illustrating one embodiment of the presentinvention;

FIG. 28 is a flow chart showing one embodiment of the present invention;

FIGS. 29A to 29C are a perspective view, a top view, and across-sectional view illustrating a structure example of a secondarybattery;

FIGS. 30A to 30D illustrate an example of a manufacturing method of asecondary battery;

FIGS. 31A, 31B, 31C1, 31C2, and 31D are a perspective view, a top view,and cross-sectional views illustrating a structure example of asecondary battery;

FIGS. 32A to 32D illustrate an example of a manufacturing method of asecondary battery;

FIG. 33 is a SEM image of a lithium-containing complex phosphate;

FIGS. 34A to 34F are SEM images of a lithium-containing complexphosphate;

FIG. 35 shows an XRD spectrum of a lithium-containing complex phosphate;and

FIG. 36 shows a histogram of particle size of a lithium-containingcomplex phosphate.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below. However,the present invention is not limited to the description below, and it iseasily understood by those skilled in the art that modes and detailsdisclosed herein can be modified in various ways. Therefore, the presentinvention is not construed as being limited to the description of theembodiments below.

Note that the terms “film” and “layer” can be interchanged with eachother depending on circumstances or conditions. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Note that in each drawing described in this specification, the size, thethickness, or the like of each component such as a positive electrode, anegative electrode, an active material layer, a separator, an exteriorbody, and the like is exaggerated for clarity in some cases. Therefore,the sizes of the components are not limited to the sizes in the drawingsand relative sizes between the components.

Note that in the structures of one embodiment of the present inventiondescribed in this specification and the like, the same portions orportions having similar functions in different drawings are denoted bythe same reference numerals, and description of such portions is notrepeated. Further, the same hatching pattern is applied to portionshaving similar functions, and the portions are not especially denoted byreference numerals in some cases.

In this specification, the power storage device is a collective termdescribing elements and devices having a power storage function. Forexample, a storage battery such as the lithium-ion secondary battery(also referred to as secondary battery), a lithium-ion capacitor, and anelectric double layer capacitor are included in the category of thepower storage device.

The descriptions in embodiments for the present invention can becombined with each other as appropriate.

Embodiment 1

In this embodiment, a method for manufacturing a lithium-containingcomplex phosphate according to one embodiment of the present inventionwill be described with reference to FIG. 1.

In a step S201 a, a lithium compound is weighed. In a step S201 b, aphosphorus compound is weighed. In a step S201 c, one or more of aniron(II) compound, a manganese(II) compound, a cobalt(II) compound, anda nickel(II) compound (hereinafter referred to as an M(II) compound) areweighed.

Typical examples of the lithium compound are lithium hydroxide-hydrate(LiOH₂O), lithium chloride (LiCl), lithium carbonate (Li₂CO₃), lithiumacetate (LiCH₃COO), and lithium oxalate ((COOLi)₂).

Typical examples of the phosphorus compound are a phosphoric acid suchas orthophosphoric acid (H₃PO₄), and ammonium hydrogenphosphates such asdiammonium hydrogenphosphate ((NH₄)₂HPO₄) and ammoniumdihydrogenphosphate (NH₄H₂PO₄).

Typical examples of the iron(II) compound are iron chloride tetrahydrate(FeCl₂.4H₂O), iron sulfate heptahydrate (FeSO₄.7H₂O), and iron acetate(Fe(CH₃COO)₂).

Typical examples of the manganese(II) compound are manganese chloridetetrahydrate (MnCl₂.4H₂O), manganese sulfate-hydrate (MnSO₄.H₂O), andmanganese acetate tetrahydrate (Mn(CH₃COO)₂.4H₂O).

Typical examples of the cobalt(II) compound are cobalt chloridehexahydrate (CoCl_(z).6H₂O), cobalt sulfate heptahydrate (CoSO₄.7H₂O),and cobalt acetate tetrahydrate (Co(CH₃COO)₂.4H₂O).

Typical examples of the nickel(II) compound are nickel chloridehexahydrate (NiCl₂.6H₂O), nickel sulfate hexahydrate (NiSO₄.6H₂O), andnickel acetate tetrahydrate (Ni(CH₃COO)₂.4H₂O).

In a step S203 a, the lithium compound is dissolved in a solvent to forma solution containing lithium. Similarly, in a step S203 b and a stepS203 c, the phosphorus compound and the M(II) compound are dissolved insolvents to form a solution containing phosphorus and a solutioncontaining M(II), respectively.

As the solvents in which the lithium compound, the phosphorus compound,and the M(II) compound are dissolved, water is given.

In a step S205, the lithium-containing solution formed in the step S203a and the phosphorus-containing solution formed in the step S203 b aremixed to form a mixed solution A in an air atmosphere.

Note that instead of the mixed solution A, a solution containing lithiumand phosphorus may be formed by dissolving a lithium salt such asLi₃PO₄, Li₂HPO₄, or LiH₂PO₄ in a solvent such as water.

In a step S207, while the mixed solution A formed in the step S205 isstirred, the M(II)-containing solution is added dropwise to the mixedsolution A little by little, so that a mixed solution B is formed in anair atmosphere.

In the step S205, a precipitate may be formed in the mixed solution Adepending on the concentrations of the lithium-containing solution andthe phosphorus-containing solution. The existence of the precipitateprevents the dropwise addition by causing clogging of a nozzle of adripping apparatus when the mixed solution A is added dropwise. In thismanner, it is preferable to drip the M(II)-containing solution little bylittle while stirring the mixed solution A in the step S207.

In a step S209, the mixed solution B is put in a container resistant toheat and pressure such as an autoclave, heated at 100° C. to 350° C.inclusive and at 0.1 MPa to 100 MPa inclusive, for 0.5 hours to 24 hoursinclusive, and then cooled. After that, the mixture in the containerresistant to heat and pressure is filtrated, the obtained solid iswashed with water, and dried.

As a result, an olivine-type lithium-containing complex phosphate(LiMPO₄ (M is one or more of Fe(II), Ni(II), Co(II), and Mn(II))) can beformed as a synthetic material A with high yield. As thelithium-containing complex phosphate, LiFePO₄, LiNiPO₄, LiCoPO₄,LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, 0<b<1),LiFe_(c)Ni_(d)Co_(e)Pa_(t), LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e<1, 0<c<1, 0<d<1, 0<e<1),LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, 0<i<1),or the like can be obtained as appropriate depending on the type of theM(II) compound. The lithium-containing complex phosphate obtained inthis embodiment might be a single-crystal grain.

Here, the shape of the lithium-containing complex phosphate obtained bythe manufacturing method shown in FIG. 1 will be described.

The schematic views of the lithium-containing complex phosphatemanufactured according to an embodiment of the present invention areshown in FIGS. 2A1 to 2A6 and 2B1 to 2B6. FIGS. 2A1 to 2A6 arebird's-eye views and FIGS. 2B1 to 2B6 are cross-sectional views. FIG.2B1 shows a cross section of FIG. 2A1, FIG. 2B2 shows a cross section ofFIG. 2A2, FIG. 2B3 shows a cross section of FIG. 2A3, FIG. 2B4 shows across section of FIG. 2A4, FIG. 2B5 shows a cross section of FIG. 2A5,and FIG. 2B6 shows a cross section of FIG. 2A6. The lithium-containingcomplex phosphate manufactured according to an embodiment of the presentinvention includes a prismatic component 204 between plate-likecomponents 200 and 202, and in some cases includes a space between theplate-like components 200 and 202 as shown in FIGS. 2A1 to 2A6 and 2B1to 2B6. Note that the prismatic component 204 between the plate-likecomponents 200 and 202 is a plurality of prismatic components in somecases as shown in FIG. 2A2 or FIG. 2B2. In FIGS. 2A1 to 2A6 and 2B1 to2B6, the components 200, 202, and 204 are differently hatched forconvenient differentiation; however, components 200, 202, and 204 mayuse the same material and may be integrated to form onelithium-containing complex phosphate. In a case where thelithium-containing complex phosphate manufactured according to anembodiment of the present invention is used as an electrode activematerial of a secondary battery, a contact area between thelithium-containing complex phosphate and an electrolytic solutionincreases by the space. The battery reaction of the secondary battery iscaused by the insertion-elimination reaction of the ions in theelectrodes and thus, when the reaction area of the positive electrodeand the lithium ion increases, the reaction speed thereof increases.Therefore, it is possible to increase the output of the secondarybattery.

When the lithium-containing complex phosphate includes a plate-likecomponent, the lithium diffusion to the whole region of the plate-likecomponent of the oxide can be completed in a short time. The shape ofthe plate-like component has a wide surface and a short side and thus,the lithium taken in from the wide surface can reach to the whole regionof the oxide by moving only a distance of the short side. Therefore, thereaction speed of the positive electrode and lithium increases. Thus, itis possible to increase the output of the secondary battery.

On a surface of the plate-like component of the lithium-containingcomplex phosphate, for example, on a surface with the largest area, atleast one of interior angles is lower than or equal to 100° and thedistance between the particles of the phosphate is increased in a layermanufactured using the phosphate. The increase of the distance betweenthe particles is preferable because the electrolytic solution entersthere easily and thus the transfer of lithium ions in the electrolyticsolution is easy.

On the other hand, when all the interior angles of the plate-likecomponent are more than or equal to 100°, the plate-like component has asubstantially regular polygon shape or circular shape and thus, theparticles are close to each other reducing the distance between theparticles in some cases. In such a case, the electrolytic solutioncannot enter the space easily; thus, inhibits the transfer of lithiumions in the electrolytic solution, and the output of the secondarybattery is suppressed in some cases. The battery reaction of thesecondary battery is caused by the insertion and elimination of the ionsinto and from the electrodes and thus, when the transfer of lithium ionsincreases, the reaction speed of the positive electrode and lithiumincreases. Therefore, it is possible to increase the output of thesecondary battery.

From such a point of view, at least one of the interior angles of theplate-like component of the phosphate is preferably less than or equalto 100°, preferably less than or equal to 95°, more preferably less thanor equal to 90°.

In the above manner, with the use of the lithium-containing complexphosphate according to one embodiment of the present invention in alithium-ion storage battery, the output of the secondary battery can beincreased.

Note that the structure described in this embodiment can be combinedwith any of the structures described in the other embodiments and theexamples as appropriate.

In Embodiment 1, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inEmbodiments 2 to 5. Note that one embodiment of the present invention isnot limited to the above examples. In other words, various embodimentsof the invention are described in this embodiment and the otherembodiments, and one embodiment of the present invention is not limitedto a particular embodiment. For example, although an example of use inthe secondary battery is described in this embodiment, one embodiment ofthe present invention is not limited thereto. Depending on circumstancesor conditions, application of one embodiment of the present invention toa variety of secondary batteries such as a lead storage battery, alithium-ion polymer secondary battery, a nickel-hydrogen battery, anickel-cadmium storage battery, a nickel-iron battery, a nickel-zincbattery, a silver oxide-zinc battery, a solid-state battery, and an airbattery is also possible. Application to a variety of power storagedevices such as a primary battery, a capacitor, and a lithium-ioncapacitor is also possible. Furthermore, depending on circumstances orconditions, for example, one embodiment of the present invention is notnecessarily applied to the secondary battery. The case wherelithium-containing complex phosphate is provided is described; however,one embodiment of the present invention is not limited to this.Depending on circumstances or conditions, it is acceptable that avariety of materials are provided in one embodiment of the presentinvention. Alternatively, for example, depending on circumstances orconditions, one embodiment of the present invention does not necessarilyinclude a lithium-containing complex phosphate.

Embodiment 2

In this embodiment, the secondary battery of one embodiment of thepresent invention and a fabricating method thereof will be described.

<<Structure and Assembly of Secondary Battery>>

A secondary battery 500 of an embodiment of the present invention isillustrated in FIG. 3A. Although FIG. 3A illustrates a mode of a thinsecondary battery as an example of the secondary battery, the secondarybattery of one embodiment of the present invention is not limited tothis example.

As illustrated in FIG. 3A, the secondary battery 500 includes a positiveelectrode 503, a negative electrode 506, a separator 507, and anexterior body 509. The secondary battery 500 may include a positiveelectrode lead 510 and a negative electrode lead 511. A bonding portion518 corresponds to a thermocompression bonding portion in the outerregion of the exterior body 509.

FIGS. 4A and 4B each illustrate an example of a cross-sectional viewalong dashed-dotted line A1-A2 in FIG. 3A. FIGS. 4A and 4B eachillustrate a cross-sectional structure of the secondary battery 500 thatis formed using a pair of the positive electrode 503 and the negativeelectrode 506.

As illustrated in FIGS. 4A and 4B, the secondary battery 500 includesthe positive electrode 503, the negative electrode 506, the separator507, an electrolytic solution 508, and the exterior body 509. Theseparator 507 is located between the positive electrode 503 and thenegative electrode 506. The electrolytic solution 508 is included in theexterior body 509.

The positive electrode 503 includes a positive electrode active materiallayer 502 and a positive electrode current collector 501. The negativeelectrode 506 includes a negative electrode active material layer 505and a negative electrode current collector 504. The active materiallayer is formed on one surface or opposite surfaces of the currentcollector. The separator 507 is located between the positive electrodecurrent collector 501 and the negative electrode current collector 504.

A battery cell may include one or more positive electrodes and one ormore negative electrodes. For example, the battery cell can have astacked-layer structure including a plurality of positive electrodes anda plurality of negative electrodes.

FIG. 5A illustrates another example of a cross-sectional view alongdashed-dotted line A1-A2 in FIG. 3A. FIG. 5B is a cross-sectional viewalong dashed-dotted line B1-B2 in FIG. 3A.

FIGS. 5A and 5B illustrate a cross-sectional structure of the secondarybattery 500 that is formed using a plurality of pairs of the positiveelectrode 503 and the negative electrode 506. There is no limitation onthe number of electrode layers of the secondary battery 500. With alarger number of electrode layers, the secondary battery can have highercapacity. In contrast, with a smaller number of electrode layers, thesecondary battery can have smaller thickness and higher flexibility.

The examples in FIGS. 5A and 5B each include two positive electrodes 503in each of which the positive electrode active material layer 502 isprovided on one surface of the positive electrode current collector 501;two positive electrodes 503 in each of which the positive electrodeactive material layers 502 are provided on opposite surfaces of thepositive electrode current collector 501; and three negative electrodes506 in each of which the negative electrode active material layers 505are provided on opposite surfaces of the negative electrode currentcollector 504. In other words, the secondary battery 500 includes sixpositive electrode active material layers 502 and six negative electrodeactive material layers 505. Note that although the separator 507 has abag-like shape in the examples illustrated in FIGS. 5A and 5B, thepresent invention is not limited to this example and the separator 507may have a strip shape or a bellows shape.

Alternatively, one positive electrode in which opposite surfaces of thepositive electrode current collector 501 are provided with the positiveelectrode active material layers 502 in FIGS. 5A and 5B is preferablyreplaced with two positive electrodes in each of which one surface ofthe positive electrode current collector 501 is provided with thepositive electrode active material layer 502. Similarly, one negativeelectrode in which opposite surfaces of the negative electrode currentcollector 504 are provided with the negative electrode active materiallayers 505 is preferably replaced with two negative electrodes in eachof which one surface of the negative electrode current collector 504 isprovided with the negative electrode active material layer 505. In thesecondary battery 500 in FIGS. 6A and 6B, surfaces of the positiveelectrode current collectors 501 on the side not provided with thepositive electrode active material layer 502 face and are in contactwith each other, and surfaces of the negative electrode currentcollectors 504 on the side not provided with the negative electrodeactive material layer 505 face and are in contact with each other. Sucha structure allows the interface between the two positive electrodecurrent collectors 501 and the two negative electrode current collectors504 to serve as sliding planes when the secondary battery 500 is curved,relieving stress caused in the secondary battery 500.

FIG. 3B illustrates the appearance of the positive electrode 503. Thepositive electrode 503 includes the positive electrode current collector501 and the positive electrode active material layer 502.

FIG. 3C illustrates the appearance of the negative electrode 506. Thenegative electrode 506 includes the negative electrode current collector504 and the negative electrode active material layer 505.

The positive electrode 503 and the negative electrode 506 preferablyinclude tab regions so that a plurality of stacked positive electrodescan be electrically connected to each other and a plurality of stackednegative electrodes can be electrically connected to each other.Furthermore, an electrode lead is preferably electrically connected tothe tab region.

As illustrated in FIG. 3B, the positive electrode 503 preferablyincludes a tab region 281. The positive electrode lead 510 is preferablywelded to part of the tab region 281. The tab region 281 preferablyincludes a region where the positive electrode current collector 501 isexposed. When the positive electrode lead 510 is welded to the regionwhere the positive electrode current collector 501 is exposed, contactresistance can be further reduced. Although FIG. 3B illustrates theexample where the positive electrode current collector 501 is exposed inthe entire tab region 281, the tab region 281 may partly include thepositive electrode active material layer 502.

As illustrated in FIG. 3C, the negative electrode 506 preferablyincludes a tab region 282. The negative electrode lead 511 is preferablywelded to part of the tab region 282. The tab region 282 preferablyincludes a region where the negative electrode current collector 504 isexposed. When the negative electrode lead 511 is welded to the regionwhere the negative electrode current collector 504 is exposed, contactresistance can be further reduced. Although FIG. 3C illustrates theexample where the negative electrode current collector 504 is exposed inthe entire tab region 282, the tab region 282 may partly include thenegative electrode active material layer 505.

Although FIG. 3A illustrates the example where the end portions of thepositive electrode 503 and the negative electrode 506 are substantiallyaligned with each other, part of the positive electrode 503 may extendbeyond the end portion of the negative electrode 506.

In the secondary battery 500, the area of a region where the negativeelectrode 506 does not overlap with the positive electrode 503 ispreferably as small as possible.

In the example illustrated in FIG. 4A, the end portion of the negativeelectrode 506 is located inward from the end portion of the positiveelectrode 503. With this structure, the entire negative electrode 506can overlap with the positive electrode 503 or the area of the regionwhere the negative electrode 506 does not overlap with the positiveelectrode 503 can be small.

The areas of the positive electrode 503 and the negative electrode 506in the secondary battery 500 are preferably substantially equal. Forexample, the areas of the positive electrode 503 and the negativeelectrode 506 that face each other with the separator 507 therebetweenare preferably substantially equal. For example, the areas of thepositive electrode active material layer 502 and the negative electrodeactive material layer 505 that face each other with the separator 507therebetween are preferably substantially equal.

For example, as illustrated in FIGS. 5A and 5B, the area of the positiveelectrode 503 on the separator 507 side is preferably substantiallyequal to the area of the negative electrode 506 on the separator 507side. When the area of a surface of the positive electrode 503 on thenegative electrode 506 side is substantially equal to the area of asurface of the negative electrode 506 on the positive electrode 503side, the region where the negative electrode 506 does not overlap withthe positive electrode 503 can be small (does not exist, ideally),whereby the secondary battery 500 can have reduced irreversiblecapacity. Alternatively, as illustrated in FIGS. 5A and 5B, the area ofthe surface of the positive electrode active material layer 502 on theseparator 507 side is preferably substantially equal to the area of thesurface of the negative electrode active material layer 505 on theseparator 507 side.

As illustrated in FIGS. 5A and 5B, the end portion of the positiveelectrode 503 and the end portion of the negative electrode 506 arepreferably substantially aligned with each other. End portions of thepositive electrode active material layer 502 and the negative electrodeactive material layer 505 are preferably substantially aligned with eachother.

In the example illustrated in FIG. 4B, the end portion of the positiveelectrode 503 is positioned on an inner side of the negative electrode506. With this structure, the entire positive electrode 503 can overlapwith the negative electrode 506 or the area of the region where thepositive electrode 503 does not overlap with the negative electrode 506can be small. In the case where the end portion of the negativeelectrode 506 is positioned inward from the end portion of the positiveelectrode 503, a current sometimes concentrates at the end portion ofthe negative electrode 506. For example, concentration of a current inpart of the negative electrode 506 results in deposition of lithium onthe negative electrode 506 in some cases. By reducing the area of theregion where the positive electrode 503 does not overlap with thenegative electrode 506, concentration of a current in part of thenegative electrode 506 can be inhibited. As a result, deposition oflithium on the negative electrode 506 can be inhibited, for example.

As illustrated in FIG. 3A, the positive electrode lead 510 is preferablyelectrically connected to the positive electrode 503. Similarly, thenegative electrode lead 511 is preferably electrically connected to thenegative electrode 506. The positive electrode lead 510 and the negativeelectrode lead 511 are exposed to the outside of the exterior body 509so as to serve as terminals for electrical contact with an externalportion.

The positive electrode current collector 501 and the negative electrodecurrent collector 504 can double as terminals for electrical contactwith an external portion. In that case, the positive electrode currentcollector 501 and the negative electrode current collector 504 may bearranged so that part of the positive electrode current collector 501and part of the negative electrode current collector 504 are exposedoutside the exterior body 509 without using electrode leads.

Although the positive electrode lead 510 and the negative electrode lead511 are provided on the same side of the secondary battery 500 in FIG.3A, the positive electrode lead 510 and the negative electrode lead 511may be provided on different sides of the secondary battery 500 asillustrated in FIG. 7. The electrode leads of the secondary battery ofone embodiment of the present invention can be freely positioned asdescribed above; therefore, the degree of freedom in design is high.Accordingly, a product including the secondary battery of one embodimentof the present invention can have a high degree of freedom in design.Furthermore, the yield of products each including the secondary batteryof one embodiment of the present invention can be increased.

<<Secondary Battery Manufacturing Method Example>>

Next, an example of a manufacturing method of the secondary battery 500,which is the secondary battery of one embodiment of the presentinvention, is described with reference to FIGS. 8A and 8B, FIGS. 9A to9C, and FIG. 10.

First, the positive electrode 503, the negative electrode 506, and theseparator 507 are stacked. Specifically, the separator 507 is positionedover the positive electrode 503. Then, the negative electrode 506 ispositioned over the separator 507. In the case of using two or morepositive electrode-negative electrode pairs, another separator 507 ispositioned over the negative electrode 506, and then, the positiveelectrode 503 is positioned. In this manner, the positive electrodes 503and the negative electrodes 506 are alternately stacked and separated bythe separator 507.

Alternatively, the separator 507 may have a bag-like shape. When theelectrode is surrounded by the separator 507, damage caused to theelectrode during a manufacturing process can be inhibited.

First, the positive electrode 503 is positioned over the separator 507.Then, the separator 507 is folded along the broken line in FIG. 8A sothat the positive electrode 503 is sandwiched by the separator 507.Although the example where the positive electrode 503 is sandwiched bythe separator 507 is described here, the negative electrode 506 may besandwiched by the separator 507.

Here, it is preferable that the outer edges of the separator 507 outsidethe positive electrode 503 be bonded so that the separator 507 has abag-like shape (or an envelope-like shape). The bonding of the outeredges of the separator 507 can be performed with the use of an adhesiveor the like, by ultrasonic welding, and by thermal fusion bonding.

Next, the outer edges of the separator 507 are bonded by heating.Bonding portions 514 are illustrated in FIG. 8A. In such a manner, thepositive electrode 503 can be covered with the separator 507.

Then, the negative electrodes 506 and the positive electrodes 503 eachcovered with the separator are alternately stacked as illustrated inFIG. 8B. Furthermore, the positive electrode lead 510 and the negativeelectrode lead 511 each having a sealing layer 115 are prepared.

After that, the positive electrode lead 510 having the sealing layer 115is connected to the tab region 281 of the positive electrode 503 asillustrated in FIG. 9A.

FIG. 9B is an enlarged view of a connection portion. The tab region 281of the positive electrode 503 and the positive electrode lead 510 areelectrically connected to each other by irradiating the bonding portion512 with ultrasonic waves while applying pressure thereto (ultrasonicwelding). In that case, a curved portion 513 is preferably provided inthe tab region 281.

This curved portion 513 can relieve stress due to external force appliedafter fabrication of the secondary battery 500. Thus, the secondarybattery 500 can have high reliability.

The tab region 282 of the negative electrode 506 and the negativeelectrode lead 511 can be electrically connected to each other by asimilar method.

Subsequently, the positive electrode 503, the negative electrode 506,and the separator 507 are positioned over the exterior body 509.

Then, the exterior body 509 is folded along a portion shown by thedotted line in the vicinity of a center portion of the exterior body 509in FIG. 9C.

In FIG. 10, the thermocompression bonding portion in the outer edges ofthe exterior body 509 is illustrated as a bonding portion 118. The outeredges of the exterior body 509 except an inlet 119 for introducing theelectrolytic solution 508 are bonded by thermocompression bonding. Inthermocompression bonding, the sealing layers provided over theelectrode leads are also melted, thereby fixing the electrode leads andthe exterior body 509 to each other. Moreover, adhesion between theexterior body 509 and the electrode leads can be increased.

After that, in a reduced-pressure atmosphere or an inert gas atmosphere,a desired amount of electrolytic solution 508 is introduced to theinside of the exterior body 509 from the inlet 119. Lastly, the inlet119 is sealed by thermocompression bonding. Through the above steps, thesecondary battery 500, which is a thin storage battery, can befabricated.

Aging is preferably performed after fabrication of the secondary battery500. The aging can be performed under the following conditions, forexample. Charging is performed at a rate of 0.001 C or more and 0.2 C orless. The temperature may be higher than or equal to room temperatureand lower than or equal to 50° C. In the case where an electrolyticsolution is decomposed and a gas is generated and accumulated in thecell, the electrolytic solution cannot be in contact with a surface ofthe electrode in some regions. That is to say, an effectual reactionarea of the electrode is reduced and effectual resistance is increased.

When the resistance is extremely increased, a charging voltage isincreased in accordance with the resistance of the electrode, and thenegative electrode potential is lowered. Consequently, lithium isintercalated into graphite and lithium is deposited on the surface ofgraphite in some cases. The lithium deposition might reduce capacity.For example, if a coating film or the like is grown on the surface afterlithium deposition, lithium deposited on the surface cannot be dissolvedagain. This lithium cannot contribute to capacity. In addition, whendeposited lithium is physically collapsed and conduction with theelectrode is lost, the lithium also cannot contribute to capacity.Therefore, the gas is preferably released to prevent the potential ofthe negative electrode from reaching the potential of lithium because ofan increase in a charging voltage.

In the case of performing degasification, for example, part of theexterior body of the thin storage battery may be cut to open the storagebattery. When the exterior body is expanded because of a gas, the formof the exterior body is preferably adjusted. Furthermore, theelectrolytic solution may be added as needed before resealing.

After the release of the gas, the charging state may be maintained at atemperature higher than room temperature, preferably higher than orequal to 30° C. and lower than or equal to 60° C., more preferablyhigher than or equal to 35° C. and lower than or equal to 50° C. for,for example, 1 hour or more and 100 hours or less. In the initialcharge, an electrolytic solution decomposed on the surface forms acoating film. The formed coating film may thus be densified when thecharging state is held at a temperature higher than room temperatureafter the release of the gas, for example.

<<Structure of Positive Electrode>>

Next, components and materials used for the secondary battery of oneembodiment of the present invention will be described. First, thepositive electrode will be described with reference to FIG. 4A. Thepositive electrode includes a positive electrode active material layer502 and a positive electrode current collector 501.

As a positive electrode active material used for the positive electrodeactive material layer 502, a material into and from which carrier ionssuch as lithium ions can be inserted and extracted can be used. Examplesof the material are a lithium-containing material with an olivinecrystal structure, a layered rock-salt crystal structure, a spinelcrystal structure, and the like.

For example, lithium iron phosphate (LiFePO₄) shown in Example 1 ispreferable because it properly has properties necessary for the positiveelectrode active material, such as safety, stability, high capacitydensity, high potential, and the existence of lithium ions which can beextracted in initial oxidation (charge).

The positive electrode active material and a negative electrode activematerial have a central role in battery reactions of a secondarybattery, and release and occlude carrier ions. To increase the lifetimeof a secondary battery, the materials preferably have a small amount ofcapacity which relates to irreversible battery reactions, and have highcharge and discharge efficiency.

The active material is in contact with an electrolytic solution. Whenthe active material reacts with the electrolytic solution, the activematerial is lost and deteriorates by the reaction, which decreases thecapacity of the secondary battery. Therefore, it is preferable that sucha reaction not be caused in the secondary battery in order to achieve asecondary battery which hardly deteriorates.

Examples of the conductive additive in the electrode include acetyleneblack (AB), graphite (black lead) particles, carbon nanotubes, reducedgraphene oxide (RGO), and fullerene.

A network for electrical conduction can be formed in the electrode bythe conductive additive. The conductive additive also allows maintainingof a path for electric conduction between the particles of the positiveelectrode active material. The addition of the conductive additive tothe positive electrode active material layer increases the electricalconductivity of the positive electrode active material layer 502.

A typical example of the binder is polyvinylidene fluoride (PVDF), andother examples of the binder include polyimide, polytetrafluoroethylene,polyvinyl chloride, ethylene-propylene-diene polymer, styrene-butadienerubber, acrylonitrile-butadiene rubber, fluorine rubber, polyvinylacetate, polymethyl methacrylate, polyethylene, and nitrocellulose.

The content of the binder with respect to the total amount of thepositive electrode active material layer 502 is preferably greater thanor equal to 1 wt % and less than or equal to 10 wt %, further preferablygreater than or equal to 2 wt % and less than or equal to 8 wt %, andstill further preferably greater than or equal to 3 wt % and less thanor equal to 5 wt %. The content of the conductive additive with respectto the total amount of the positive electrode active material layer 502is preferably greater than or equal to 1 wt % and less than or equal to10 wt %, further preferably greater than or equal to 1 wt % and lessthan or equal to 5 wt %.

In the case where the positive electrode active material layer 502 isformed by a coating method, the positive electrode active material, thebinder, the conductive additive, and a dispersion medium are mixed toform an electrode slurry, and the slurry is applied to the positiveelectrode current collector 501 and dried.

The positive electrode current collector 501 can be formed using amaterial, which has high conductivity and is not alloyed with carrierions of lithium or the like, such as stainless steel, gold, platinum,aluminum, or titanium, or an alloy thereof. Alternatively, an aluminumalloy to which an element which improves heat resistance, such assilicon, titanium, neodymium, scandium, or molybdenum, is added can beused. Still alternatively, a metal element which forms silicide byreacting with silicon can be used. Examples of the metal element whichforms silicide by reacting with silicon are zirconium, titanium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,cobalt, nickel, and the like. The positive electrode current collectorcan have a foil shape, a plate (sheet) shape, a net shape, apunching-metal shape, an expanded-metal shape, or the like asappropriate.

Through the above steps, the positive electrode of the secondary batterycan be formed.

<Structure of Negative Electrode>

Next, the negative electrode is described with reference to FIG. 4A. Thenegative electrode includes a negative electrode active material layer505 and a negative electrode current collector 504. Steps of forming thenegative electrode are described below.

Examples of a carbon-based material as the negative electrode activematerial used for the negative electrode active material layer 505include graphite, graphitizing carbon (soft carbon), non-graphitizingcarbon (hard carbon), a carbon nanotube, graphene, and carbon black.Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, pitch-basedartificial graphite, and natural graphite such as spherical naturalgraphite. In addition, the shape of the graphite is a flaky shape and aspherical shape, for example.

Other than the carbon-based material, a material which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used as the negative electrode activematerial. A material including at least one of Ga, Si, Al, Ge, Sn, Pb,Sb, Bi, Ag, Zn, Cd, In, and the like can be used, for example. Suchelements have higher capacity than carbon. In particular, silicon has asignificantly high theoretical capacity of 4200 mAh/g. Examples of analloy-based material (compound-based material) using such elementsinclude Mg₂Si, Mg₂Ge, Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅,Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn.

Alternatively, for the negative electrode active material, an oxide suchas SiO, SnO, SnO₂, titanium dioxide (TiO₂), lithium titanium oxide(Li₄Ti₅O₁₂), lithium-graphite intercalation compound (Li_(x)C₆), niobiumpentoxide (Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) canbe used.

Still alternatively, for the negative electrode active material,Li_(3-x)M_(x)N (M is Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

When a nitride containing lithium and a transition metal is used,lithium is included in the negative electrode active material; thus, thenegative electrode active material can be used in combination with amaterial for a positive electrode active material which does not containlithium, such as V₂O₅ or Cr₃O₈. In the case of using a materialcontaining lithium as a positive electrode active material, the nitridecontaining lithium and a transition metal can be used for the negativeelectrode active material by extracting lithium contained in thepositive electrode active material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active material; for example, a transitionmetal oxide with which an alloying reaction with lithium is not caused,such as cobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), maybe used for the negative electrode active material. Other examples ofthe material which causes a conversion reaction include oxides such asFe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides such as CoS_(0.89), NiS, orCuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄, phosphides such as NiP₂,FeP₂, and CoP₃, and fluorides such as FeF₃ and BiF₃.

The particle size of the negative electrode active material ispreferably greater than or equal to 50 nm and less than or equal to 100μm, for example.

Note that it is acceptable that a plurality of materials for activematerials are combined at a given proportion both for the positiveelectrode active material layer 502 and the negative electrode activematerial layer 505. The use of a plurality of materials for the activematerial layer makes it possible to select the performance of the activematerial layer in detail.

Examples of the conductive additive in the electrode include acetyleneblack (AB), graphite (black lead) particles, carbon nanotubes, reducedgraphene oxide (RGO), and fullerene.

A network for electrical conduction can be formed in the electrode bythe conductive additive. The conductive additive also allows maintainingof a path for electric conduction between the particles of the negativeelectrode active material. The addition of the conductive additive tothe negative electrode active material layer increases the electricconductivity of the negative electrode active material layer 505.

A typical example of the binder is polyvinylidene fluoride (PVDF), andother examples of the binder include polyimide, polytetrafluoroethylene,polyvinyl chloride, ethylene-propylene-diene polymer, styrene-butadienerubber, acrylonitrile-butadiene rubber, fluorine rubber, polyvinylacetate, polymethyl methacrylate, polyethylene, and nitrocellulose.

The content of the binder with respect to the total amount of thenegative electrode active material layer 505 is preferably greater thanor equal to 1 wt % and less than or equal to 10 wt %, further preferablygreater than or equal to 2 wt % and less than or equal to 8 wt %, andstill further preferably greater than or equal to 3 wt % and less thanor equal to 5 wt %. The content of the conductive additive with respectto the total amount of the negative electrode active material layer 505is preferably greater than or equal to 1 wt % and less than or equal to10 wt %, further preferably greater than or equal to 1 wt % and lessthan or equal to 5 wt %.

Next, the negative electrode active material layer 505 is formed on thenegative electrode current collector 504. In the case where the negativeelectrode active material layer 505 is formed by a coating method, thenegative electrode active material, the binder, the conductive additive,and a dispersion medium are mixed to form a slurry, and the slurry isapplied to the negative electrode current collector 504 and dried. Ifnecessary, pressing may be performed after the drying.

The negative electrode current collector 504 can be formed using amaterial, which has high conductivity and is not alloyed with carrierions of lithium or the like, such as stainless steel, gold, platinum,zinc, iron, copper, titanium, or tantalum, or an alloy thereof.Alternatively, a metal element which forms silicide by reacting withsilicon can be used. Examples of the metal element which forms silicideby reacting with silicon include zirconium, titanium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, andthe like. The negative electrode current collector 504 can have a foilshape, a plate (sheet) shape, a net shape, a cylindrical shape, a coilshape, a punching-metal shape, an expanded-metal shape, or the like asappropriate. The negative electrode current collector 504 preferably hasa thickness greater than or equal to 5 μm and less than or equal to 30μm. A part of the surface of the electrode current collector may beprovided with an undercoat layer using graphite or the like.

Through the above steps, the negative electrode of the secondary batterycan be formed.

<<Structure of Separator>>

The separator 507 will be described. The separator 507 may be formedusing a material such as paper, nonwoven fabric, fiberglass, syntheticfiber such as nylon (polyamide), vinylon (polyvinyl alcohol basedfiber), polyester, acrylic, polyolefin, or polyurethane. However, amaterial that does not dissolve in an electrolytic solution describedlater needs to be selected.

More specifically, as a material for the separator 507, high-molecularcompounds based on fluorine-based polymer, polyether such aspolyethylene oxide and polypropylene oxide, polyolefin such aspolyethylene and polypropylene, polyacrylonitrile, polyvinylidenechloride, polymethyl methacrylate, polymethylacrylate, polyvinylalcohol, polymethacrylonitrile, polyvinyl acetate, polyvinylpyrrolidone,polyethyleneimine, polybutadiene, polystyrene, polyisoprene, andpolyurethane, derivatives thereof, cellulose, paper, nonwoven fabric,and a glass fiber can be used either alone or in combination.

<<Components of Electrolytic Solution>>

The electrolytic solution 508 used in the secondary battery of oneembodiment of the present invention is preferably a nonaqueous solution(solvent) containing an electrolyte (solute).

For a solvent of the electrolytic solution 508, a material in whichcarrier ions can transfer is used. For example, an aprotic organicsolvent is preferably used, and one of ethylene carbonate (EC),propylene carbonate (PC), butylene carbonate, chloroethylene carbonate,vinylene carbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate(DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methylformate, methyl acetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

When a gelled high-molecular material is used as the solvent of theelectrolytic solution 508, safety against liquid leakage and the like isimproved. Furthermore, the secondary battery can be thinner and morelightweight. Typical examples of the gelled polymer material include asilicone gel, an acrylic gel, an acrylonitrile gel, a polyethyleneoxide-based gel, a polypropylene oxide-based gel, a fluorine-basedpolymer gel, and the like.

Alternatively, the use of one or more of ionic liquids (room temperaturemolten salts) that have non-flammability and non-volatility as thesolvent for the electrolytic solution can prevent the secondary batteryfrom exploding or catching fire even when the secondary batteryinternally shorts out or the internal temperature increases due toovercharging or the like. Thus, the secondary battery has improvedsafety.

Although the case where carrier ions are lithium ions in the aboveelectrolyte is described, carrier ions other than lithium ions can beused. When the carrier ions other than lithium ions are alkali metalions or alkaline-earth metal ions, instead of lithium in the lithiumsalts, an alkali metal (e.g., sodium or potassium) or an alkaline-earthmetal (e.g., calcium, strontium, barium, beryllium, or magnesium) may beused as the electrolyte.

The electrolytic solution used for the secondary battery is preferably ahighly purified one so as to contain a negligible small amount of dustparticles and elements other than the constituent elements of theelectrolytic solution (hereinafter, also simply referred to asimpurities). Specifically, the mass ratio of impurities to theelectrolytic solution is less than or equal to 1%, preferably less thanor equal to 0.1%, and further preferably less than or equal to 0.01%. Anadditive agent such as vinylene carbonate may be added to theelectrolytic solution.

In the case of using lithium ions as carrier ions, as an electrolytedissolved in the above solvent, one of lithium salts such as LiPF₆,LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀Li₂B₁₂C1 ₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃,LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂) (CF₃SO₂), and LiN(C₂F₅SO₂)₂ can be used, ortwo or more of these lithium salts can be used in an appropriatecombination in an appropriate ratio.

Note that the electrolytic solution reacts with and corrodes thepositive electrode current collector in some cases. In order to preventsuch corrosion, several weight percent of LiPF₆ may be added to theelectrolytic solution, in which case a passive film is formed on asurface of the positive electrode current collector and may be able toprevent reaction between the electrolytic solution and the positiveelectrode current collector. Note that the concentration of LiPF₆ isless than or equal to 10 wt %, preferably less than or equal to 5 wt %,and further preferably less than or equal to 3 wt % in order that thepositive electrode active material layer is not dissolved.

<<Graphene Compound>>

Note that in one embodiment of the present invention, a graphenecompound can be used in a component of the power storage device. Asdescribed later, when modification is performed, the structure andcharacteristics of a graphene compound can be selected from a widerrange of alternatives. Thus, a preferable property can be exhibited inaccordance with a component in which a graphene compound is to be used.Moreover, a graphene compound has high mechanical strength and thereforecan be used in a component of a flexible power storage device. Graphenecompounds are described below.

Graphene has carbon atoms arranged in one atomic layer. A π bond existsbetween the carbon atoms. Graphene including two or more and one hundredor less layers is referred to as multilayer graphene in some cases. Thelength in the longitudinal direction or the length of the major axis ina surface in each of graphene and multilayer graphene is greater than orequal to 50 nm and less than or equal to 100 μm or greater than or equalto 800 nm and less than or equal to 50 μm.

In this specification and the like, a compound including graphene ormultilayer graphene as a basic skeleton is referred to as a graphenecompound. Graphene compounds include graphene and multilayer graphene.

Graphene compounds are described below.

A graphene compound is, for example, a compound where graphene ormultilayer graphene is modified with an atom other than carbon or anatomic group with an atom other than carbon. A graphene compound may bea compound where graphene or multilayer graphene is modified with anatomic group composed mainly of carbon, such as an alkyl group or analkylene group. An atomic group that modifies graphene or multilayergraphene is referred to as a substituent, a functional group, acharacteristic group, or the like in some cases. Modification in thisspecification and the like refers to introduction of an atom other thancarbon, an atomic group with an atom other than carbon, or an atomicgroup composed mainly of carbon to graphene, multilayer graphene, agraphene compound, or graphene oxide (described later) by a substitutionreaction, an addition reaction, or other reactions.

Note that the surface side and the rear surface side of graphene may bemodified with different atoms or atomic groups. In multilayer graphene,multiple layers may be modified with different atoms or atomic groups.

An example of the above graphene modified with an atom or an atomicgroup is graphene or multilayer graphene that is modified with oxygen ora functional group containing oxygen. Examples of a functional groupcontaining oxygen include an epoxy group, a carbonyl group such as acarboxyl group, and a hydroxyl group. A graphene compound modified withoxygen or a functional group containing oxygen is referred to asgraphene oxide in some cases. In this specification, graphene oxidesinclude multilayer graphene oxides.

As an example of modification of graphene oxide, silylation of grapheneoxide will be described. First, in a nitrogen atmosphere, graphene oxideis put in a container, n-butylamine (C₄H₉NH₂) is added to the container,and stirring is performed for one hour with the temperature kept at 60°C. Then, toluene is added to the container, alkyltrichlorosilane isadded thereto as a silylating agent, and stirring is performed in anitrogen atmosphere for five hours with the temperature kept at 60° C.Then, toluene is further added to the container, and the resultingsolution is suction-filtrated to give a solid powder. The powder isdispersed in ethanol, and the resulting solution is suction-filtered togive a solid powder. The powder is dispersed in acetone, and theresulting solution is suction-filtered to give a solid powder. A liquidof the solid powder is vaporized to give silylated graphene oxide.

The modification is not limited to silylation, and silylation is notlimited to the above method. The modification is not limited tointroduction of an atom or an atomic group of one kind, and themodification of two or more types may be performed to introduce atoms oratomic groups of two or more kinds. By introducing a given atomic groupto a graphene compound, the physical property of the graphene compoundcan be changed. Therefore, by performing desirable modification inaccordance with the application of a graphene compound, a desiredproperty of the graphene compound can be exhibited intentionally.

A formation method example of graphene oxide is described below.Graphene oxide can be obtained by oxidizing the above graphene ormultilayer graphene. Alternatively, graphene oxide can be obtained bybeing separated from graphite oxide. Graphite oxide can be obtained byoxidizing graphite. The graphene oxide may be further modified with theabove atom or atomic group.

A compound that can be obtained by reducing graphene oxide is referredto as reduced graphene oxide (RGO) in some cases. In RGO, in some cases,all oxygen atoms contained in the graphene oxide are not extracted andpart of them remains in a state where oxygen or atomic group containingoxygen that is bonded to carbon. In some cases, RGO includes afunctional group, e.g., an epoxy group, a carbonyl group such as acarboxyl group, or a hydroxyl group.

A graphene compound may have a sheet-like shape where a plurality ofgraphene compounds partly overlap with each other. Such a graphenecompound is referred to as a graphene compound sheet in some cases. Thegraphene compound sheet has, for example, an area with a thicknessgreater than or equal to 0.33 nm and smaller than or equal to 10 mm,preferably greater than 0.34 nm and less than or equal to 10 μm. Thegraphene compound sheet may be modified with an atom other than carbon,an atomic group containing an atom other than carbon, an atomic groupcomposed mainly of carbon such as an alkyl group, or the like. Aplurality of layers in the graphene compound sheet may be modified withdifferent atoms or atomic groups.

A graphene compound may have a five-membered ring composed of carbon ora poly-membered ring that is a seven- or more-membered ring composed ofcarbon, in addition to a six-membered ring composed of carbon. In theneighborhood of a poly-membered ring which is a seven- or more-memberedring, a region through which a lithium ion can pass may be generated.

A plurality of graphene compounds may be gathered to form a sheet-likeshape.

A graphene compound has a planar shape, thereby enabling surfacecontact.

In some cases, a graphene compound has high conductivity even when it isthin. The contact area between graphene compounds or between a graphenecompound and an active material can be increased by surface contact.Thus, even with a small amount of a graphene compound per volume, aconductive path can be formed efficiently.

In contrast, a graphene compound may also be used as an insulator. Forexample, a graphene compound sheet may be used as a sheet-likeinsulator. Graphene oxide, for example, has a higher insulation propertythan a graphene compound that is not oxidized, in some cases. A graphenecompound modified with an atomic group may have an improved insulationproperty, depending on the type of the modifying atomic group.

A graphene compound in this specification and the like may include aprecursor of graphene. The precursor of graphene refers to a substanceused for forming graphene. The precursor of graphene may contain theabove graphene oxide, graphite oxide, or the like.

Graphene containing an alkali metal or an element other than carbon,such as oxygen, is referred to as a graphene analog in some cases. Inthis specification and the like, graphene compounds include grapheneanalogs.

A graphene compound in this specification and the like may include anatom, an atomic group, and ions thereof between the layers. The physicalproperties, such as electric conductivity and ion conductivity, of agraphene compound sometimes change when an atom, an atomic group, andions thereof exist between layers of the compound. In addition, adistance between the layers is increased in some cases.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. A modified graphene compound canhave extremely low conductivity and serve as an insulator depending onthe type of the modification. A graphene compound has a planar shape. Agraphene compound enables low-resistance surface contact.

<<Structure of Exterior Body>>

Next, the exterior body 509 will be described. As the exterior body 509,a film having a three-layer structure can be used, for example. In thethree-layer structure, a highly flexible metal thin film of, forexample, aluminum, stainless steel, copper, and nickel is provided overa film formed of a material such as polyethylene, polypropylene,polycarbonate, ionomer, and polyamide, and an insulating synthetic resinfilm of, for example, a polyamide-based resin or a polyester-based resinis provided as the outer surface of the exterior body over the metalthin film. With such a three-layer structure, permeation of theelectrolytic solution and a gas can be blocked and an insulatingproperty and resistance to the electrolytic solution can be provided.The exterior body is folded inside in two, or two exterior bodies arestacked with the inner surfaces facing each other, in which caseapplication of heat melts the materials on the overlapping innersurfaces to cause fusion bonding between the two exterior bodies. Inthis manner, a sealing structure can be formed.

A portion where the sealing structure is formed by fusion bonding or thelike of the exterior body is referred to as a sealing portion. In thecase where the exterior body is folded inside in two, the sealingportion is formed in the place other than the fold, and a first regionof the exterior body and a second region of the exterior body thatoverlaps with the first region are fusion-bonded, for example. In thecase where two exterior bodies are stacked, the sealing portion isformed along the entire circumference by heat fusion bonding or thelike.

<<Flexible Secondary Battery>>

When a flexible material is selected from materials of the componentsdescribed in this embodiment and used, a flexible secondary battery canbe fabricated. Deformable devices are currently under active researchand development. For such devices, flexible secondary batteries aredemanded.

In the case of bending a secondary battery in which a battery material1805 including electrodes and an electrolytic solution is sandwichedbetween two films as exterior bodies, a radius 1802 of curvature of afilm 1801 on the side closer to a center 1800 of curvature of thesecondary battery is smaller than a radius 1804 of curvature of a film1803 on the side further from the center 1800 of curvature (FIG. 11A).When the secondary battery is curved and has an arc-shaped crosssection, compressive stress is applied to a surface of the film close tothe center 1800 of curvature and tensile stress is applied to a surfaceof the film far from the center 1800 of curvature (FIG. 11B).

When the flexible secondary battery is deformed, strong stress isapplied to the exterior bodies. However, even with the compressivestress and tensile stress due to the deformation of the secondarybattery, the influence of a strain can be reduced by forming a patternincluding projections or depressions on surfaces of the exterior bodies.For this reason, the secondary battery can change its form in such arange that the exterior body on the side closer to the center ofcurvature has a curvature radius of 30 mm, preferably 10 mm.

The radius of curvature of a surface will be described with reference toFIGS. 12A to 12C. In FIG. 12A, on a plan surface 1701 along which acurved surface 1700 is cut, part of a curve 1702 of the curved surface1700 is approximate to an arc of a circle, and the radius of the circleis referred to as a radius of curvature 1703 and the center of thecircle is referred to as a center of curvature 1704. FIG. 12B is a topview of the curved surface 1700. FIG. 12C is a cross-sectional view ofthe curved surface 1700 taken along the plan surface 1701. When a curvedsurface is cut by a plan surface, the radius of curvature of a curve ina cross section differs depending on the angle between the curvedsurface and the plan surface or on the cut position, and the smallestradius of curvature is defined as the radius of curvature of a surfacein this specification and the like.

Note that the cross-sectional shape of the secondary battery is notlimited to a simple arc shape, and the cross section can be partlyarc-shaped; for example, a shape illustrated in FIG. 11C, a wavy shapeillustrated in FIG. 11D, or an S shape can be used. When the curvedsurface of the secondary battery has a shape with a plurality of centersof curvature, the secondary battery can change its form in such a rangethat a curved surface with the smallest radius of curvature among radiiof curvature with respect to the plurality of centers of curvature,which is a surface of the exterior body on the side closer to the centerof curvature, has a curvature radius of 30 mm, preferably 10 mm.

As the positive electrode active material layer of the secondary batteryaccording to this embodiment, the positive electrode active materiallayer according to one embodiment of the present invention is used. Forthis reason, the output of the secondary battery can be increased.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and example.

Embodiment 3

In this embodiment, structures of a secondary battery of one embodimentof the present invention are described with reference to FIGS. 13A to13C and FIGS. 14A and 14B.

<<Coin-Type Secondary Battery>>

FIG. 13A is an external view of a coin-type (single-layer flat type)secondary battery. FIG. 13B is a cross-sectional view thereof.

In a coin-type secondary battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. The positiveelectrode active material layer 306 may further include a binder forincreasing adhesion of positive electrode active materials, a conductiveadditive for increasing the conductivity of the positive electrodeactive material layer, and the like in addition to the positiveelectrode active materials.

As a positive electrode active material, the positive electrode activematerial described in Embodiment 1 can be used.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308. The negativeelectrode active material layer 309 may further include a binder forincreasing adhesion of negative electrode active materials, a conductiveadditive for increasing the conductivity of the negative electrodeactive material layer, and the like in addition to the negativeelectrode active materials. A separator 310 and an electrolyte (notillustrated) are provided between the positive electrode active materiallayer 306 and the negative electrode active material layer 309.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolytic solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel or thelike) can be used. Alternatively, the positive electrode can 301 and thenegative electrode can 302 are preferably covered with nickel, aluminum,or the like in order to prevent corrosion due to the electrolyticsolution. The positive electrode can 301 and the negative electrode can302 are electrically connected to the positive electrode 304 and thenegative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolytic solution. Then, asillustrated in FIG. 13B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type secondarybattery 300 can be fabricated.

Here, a current flow in charging the secondary battery will be describedwith reference to FIG. 13C. When a secondary battery using lithium isregarded as a closed circuit, lithium ions transfer and a current flowsin the same direction. Note that in the secondary battery using lithium,an anode and a cathode change places in charge and discharge, and anoxidation reaction and a reduction reaction occur on the correspondingsides; hence, an electrode with a high redox potential is called apositive electrode and an electrode with a low redox potential is calleda negative electrode. For this reason, in this specification, thepositive electrode is referred to as a “positive electrode” or a “pluselectrode” and the negative electrode is referred to as a “negativeelectrode” or a “minus electrode” in all the cases where charging isperformed, discharging is performed, a reverse pulse current issupplied, and a charging current is supplied. The use of the terms“anode” and “cathode” related to an oxidation reaction and a reductionreaction might cause confusion because the anode and the cathode changeplaces at the time of charging and discharging. Thus, the terms “anode”and “cathode” are not used in this specification. If the term “anode” or“cathode” is used, whether it is at the time of charging or dischargingis noted and whether it corresponds to a positive electrode or anegative electrode is also noted.

Two terminals in FIG. 13C are connected to a charger, and a secondarybattery 400 is charged. As the charge of the secondary battery 400proceeds, a potential difference between electrodes increases. Thepositive direction in FIG. 13C is the direction in which a current flowsfrom the one terminal outside the secondary battery 400 to a positiveelectrode 402, flows from the positive electrode 402 to a negativeelectrode 404 in the secondary battery 400, and flows from the negativeelectrode 404 to the other terminal outside the secondary battery 400.In other words, the direction of a charging current is the direction ofa flow of a current.

<<Cylindrical Secondary Battery>>

Next, an example of a cylindrical secondary battery will be describedwith reference to FIGS. 14A and 14B. As illustrated in FIG. 14A, acylindrical secondary battery 600 includes a positive electrode cap(battery cap) 601 on the top surface and a battery can (outer can) 602on the side surface and bottom surface. The positive electrode cap 601and the battery can 602 are insulated from each other by a gasket(insulating gasket) 610.

FIG. 14B is a diagram schematically illustrating a cross section of thecylindrical secondary battery. Inside the battery can 602 having ahollow cylindrical shape, a battery element in which a strip-likepositive electrode 604 and a strip-like negative electrode 606 are woundwith a strip-like separator 605 interposed therebetween is provided.Although not illustrated, the battery element is wound around a centerpin. One end of the battery can 602 is closed and the other end thereofis open. For the battery can 602, a metal having a corrosion-resistantproperty to an electrolytic solution, such as nickel, aluminum, ortitanium, an alloy of such a metal, or an alloy of such a metal andanother metal (e.g., stainless steel or the like) can be used.Alternatively, the battery can 602 is preferably covered with nickel,aluminum, or the like in order to prevent corrosion due to theelectrolytic solution. Inside the battery can 602, the battery elementin which the positive electrode, the negative electrode, and theseparator are wound is provided between a pair of insulating plates 608and 609 which face each other. Furthermore, a nonaqueous electrolyticsolution (not illustrated) is injected inside the battery can 602provided with the battery element. As the nonaqueous electrolyticsolution, a nonaqueous electrolytic solution that is similar to that ofthe coin-type secondary battery can be used.

Although the positive electrode 604 and the negative electrode 606 canbe formed in a manner similar to that of the positive electrode and thenegative electrode of the coin-type secondary battery described above,the difference lies in that, since the positive electrode and thenegative electrode of the cylindrical secondary battery are wound,active materials are formed on both sides of the current collectors. Apositive electrode terminal (positive electrode current collecting lead)603 is connected to the positive electrode 604, and a negative electrodeterminal (negative electrode current collecting lead) 607 is connectedto the negative electrode 606. Both the positive electrode terminal 603and the negative electrode terminal 607 can be formed using a metalmaterial such as aluminum. The positive electrode terminal 603 and thenegative electrode terminal 607 are resistance-welded to a safety valvemechanism 612 and the bottom of the battery can 602, respectively. Thesafety valve mechanism 612 is electrically connected to the positiveelectrode cap 601 through a positive temperature coefficient (PTC)element 611. The safety valve mechanism 612 cuts off electricalconnection between the positive electrode cap 601 and the positiveelectrode 604 when the internal pressure of the battery exceeds apredetermined threshold value. The PTC element 611, which serves as athermally sensitive resistor whose resistance increases as temperaturerises, limits the amount of current by increasing the resistance, inorder to prevent abnormal heat generation. Note that barium titanate(BaTiO₃)-based semiconductor ceramic can be used for the PTC element.

As the positive electrode active material layer of the secondary batteryaccording to this embodiment, the positive electrode active materiallayer according to one embodiment of the present invention is used. Forthis reason, the output of the secondary battery can be increased.

FIGS. 15A to 15E illustrate examples of electronic devices includingflexible laminated secondary batteries. Examples of electronic deviceseach including a flexible power storage device include televisiondevices (also referred to as televisions or television receivers),monitors of computers or the like, cameras such as digital cameras anddigital video cameras, digital photo frames, mobile phones (alsoreferred to as mobile phones or mobile phone devices), portable gamemachines, portable information terminals, audio reproducing devices, andlarge game machines such as pachinko machines.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of a car.

FIG. 15A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 15B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is bent by the external force, the power storagedevice 7407 included in the mobile phone 7400 is also bent. FIG. 15Cillustrates the bent power storage device 7407. The power storage device7407 is a laminated secondary battery.

FIG. 15D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 15Eillustrates the bent power storage device 7104.

<<Structural Example of Power Storage Device>>

Structural examples of power storage devices will be described withreference to FIGS. 16A and 16B, FIGS. 17A1, 17A2, 17B1, and 17B2, FIGS.18A and 18B, FIGS. 19A and 19B, and FIG. 20.

FIGS. 16A and 16B are external views of a power storage device. Thepower storage device includes a circuit board 900 and a secondarybattery 913. A label 910 is attached to the secondary battery 913. Asshown in FIG. 16B, the power storage device further includes a terminal951, a terminal 952, an antenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. The shape of each of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape. Furthermore, aplanar antenna, an aperture antenna, a traveling-wave antenna, an EHantenna, a magnetic-field antenna, a dielectric antenna, or the like maybe used. Alternatively, the antenna 914 or the antenna 915 may be aflat-plate conductor. The flat-plate conductor can serve as one ofconductors for electric field coupling. That is, the antenna 914 or theantenna 915 can serve as one of two conductors of a capacitor. Thus,electric power can be transmitted and received not only by anelectromagnetic field or a magnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage device includes a layer 916 between the secondarybattery 913 and the antennas 914 and 915. The layer 916 may have afunction of preventing an adverse effect on an electromagnetic field bythe secondary battery 913. As the layer 916, for example, a magneticbody can be used.

Note that the structure of the power storage device is not limited tothat shown in FIGS. 16A and 16B.

For example, as shown in FIGS. 17A1 and 17A2, two opposite surfaces ofthe secondary battery 913 in FIGS. 16A and 16B may be provided withrespective antennas. FIG. 17A1 is an external view showing one side ofthe opposite surfaces, and FIG. 17A2 is an external view showing theother side of the opposite surfaces. For portions similar to those inFIGS. 16A and 16B, a description of the power storage device illustratedin FIGS. 16A and 16B can be referred to as appropriate.

As illustrated in FIG. 17A1, the antenna 914 is provided on one of theopposite surfaces of the secondary battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 17A2, the antenna915 is provided on the other of the opposite surfaces of the secondarybattery 913 with a layer 917 interposed therebetween. The layer 917 mayhave a function of preventing an adverse effect on an electromagneticfield by the secondary battery 913, for example. As the layer 917, forexample, a magnetic body can be used.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as illustrated in FIGS. 17B1 and 17B2, two oppositesurfaces of the secondary battery 913 in FIGS. 16A and 16B may beprovided with different types of antennas. FIG. 17B1 is an external viewshowing one side of the opposite surfaces, and FIG. 17B2 is an externalview showing the other side of the opposite surfaces. For portionssimilar to those in FIGS. 16A and 16B, a description of the powerstorage device illustrated in FIGS. 16A and 16B can be referred to asappropriate.

As illustrated in FIG. 17B1, the antennas 914 and 915 are provided onone of the opposite surfaces of the secondary battery 913 with the layer916 interposed therebetween, and as illustrated in FIG. 17B2, an antenna918 is provided on the other of the opposite surfaces of the secondarybattery 913 with the layer 917 interposed therebetween. The antenna 918has a function of communicating data with an external device, forexample. An antenna with a shape that can be applied to the antennas 914and 915, for example, can be used as the antenna 918. As a system forcommunication using the antenna 918 between the power storage device andanother device, a response method that can be used between the powerstorage device and another device, such as NFC, can be employed.

Alternatively, as illustrated in FIG. 18A, the secondary battery 913 inFIGS. 16A and 16B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. For portions similar to thosein FIGS. 16A and 16B, a description of the power storage deviceillustrated in FIGS. 16A and 16B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charging is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 18B, the secondary battery 913illustrated in FIGS. 16A and 16B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 16A and 16B, a descriptionof the power storage device illustrated in FIGS. 16A and 16B can bereferred to as appropriate.

The sensor 921 has a function of measuring, for example, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, electric current, voltage,electric power, radiation, flow rate, humidity, gradient, oscillation,odor, or infrared rays. With the sensor 921, for example, data on anenvironment (e.g., temperature) where the power storage device is placedcan be determined and stored in a memory inside the circuit 912.

Furthermore, structural examples of the secondary battery 913 will bedescribed with reference to FIGS. 19A and 19B and FIG. 20.

The secondary battery 913 illustrated in FIG. 19A includes a wound body950 provided with the terminals 951 and 952 inside a housing 930. Thewound body 950 is soaked in an electrolytic solution inside the housing930. The terminal 952 is in contact with the housing 930. An insulatoror the like prevents contact between the terminal 951 and the housing930. Note that in FIG. 19A, the housing 930 divided into two pieces isillustrated for convenience; however, in the actual structure, the woundbody 950 is covered with the housing 930 and the terminals 951 and 952extend to the outside of the housing 930. For the housing 930, a metalmaterial (e.g., aluminum) or a resin material can be used.

Note that as illustrated in FIG. 19B, the housing 930 in FIG. 19A may beformed using a plurality of materials. For example, in the secondarybattery 913 in FIG. 19B, a housing 930 a and a housing 930 b are bondedto each other and the wound body 950 is provided in a region surroundedby the housing 930 a and the housing 930 b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, blocking of an electricfield by the secondary battery 913 can be prevented. When an electricfield is not significantly blocked by the housing 930 a, an antenna suchas the antennas 914 and 915 may be provided inside the housing 930 a.For the housing 930 b, a metal material can be used, for example.

FIG. 20 illustrates the structure of the wound body 950. The wound body950 includes a negative electrode 931, a positive electrode 932, andseparators 933. The wound body 950 is obtained by winding a sheet of astack in which the negative electrode 931 overlaps with the positiveelectrode 932 with the separator 933 provided therebetween. Note that aplurality of stacks of the negative electrode 931, the positiveelectrode 932, and the separator 933 may be stacked.

The negative electrode 931 is connected to the terminal 911 in FIGS. 16Aand 16B via one of the terminals 951 and 952. The positive electrode 932is connected to the terminal 911 in FIGS. 16A and 16B via the other ofthe terminals 951 and 952.

[Examples of Electric Devices: Vehicles]

Next, examples where a secondary battery is used in a vehicle will bedescribed. The use of secondary batteries in vehicles enables productionof next-generation clean energy vehicles such as hybrid electricvehicles (HEV s), electric vehicles (EVs), and plug-in hybrid electricvehicles (PHEVs).

FIGS. 21A and 21B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8100 illustrated inFIG. 21A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8100 is a hybrid electric vehiclecapable of driving appropriately using either the electric motor or theengine. One embodiment of the present invention can provide a vehiclewhich can be repeatedly charged and discharged. The automobile 8100includes the power storage device. The power storage device is used notonly for driving an electric motor, but also for supplying electricpower to a light-emitting device such as a headlight 8101 or an interiorlight (not illustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8100. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8100, such as a navigation system.

FIG. 21B illustrates an automobile 8200 including the power storagedevice. The automobile 8200 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.21B, the power storage device included in the automobile 8200 is chargedwith the use of a ground-based charging apparatus 8021 through a cable8022. The charging apparatus 8021 may be a charging station provided ina commerce facility or a power source in a house. For example, with theuse of a plug-in technique, the power storage device included in theautomobile 8200 can be charged by being supplied with electric powerfrom outside. The charging can be performed by converting AC electricpower into DC electric power through a converter such as an AC-DCconverter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

Of one embodiment of the present invention, the power storage device canhave improved cycle life and reliability. Furthermore, of one embodimentof the present invention, the power storage device itself can be mademore compact and lightweight as a result of improved characteristics ofthe power storage device. The compact and lightweight power storagedevice contributes to a reduction in the weight of a vehicle, and thusincreases the mileage. Furthermore, the power storage device included inthe vehicle can be used as a power source for supplying electric powerto products other than the vehicle. In such a case, the use of acommercial power source can be avoided at peak time of electric powerdemand.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Note that in the case where at least one specific example is describedin a diagram or a text described in one embodiment in this specificationand the like, it will be readily appreciated by those skilled in the artthat a broader concept of the specific example can be derived.Therefore, in the diagram or the text described in one embodiment, inthe case where at least one specific example is described, a broaderconcept of the specific example is disclosed as one embodiment of theinvention, and one embodiment of the invention can be constituted. Theembodiment of the invention is clear.

Note that in this specification and the like, a content described in atleast a diagram (which may be part of the diagram) is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. Therefore, when a certain content is described in adiagram, the content is disclosed as one embodiment of the inventioneven when the content is not described with a text, and one embodimentof the invention can be constituted. In a similar manner, part of adiagram, which is taken out from the diagram, is disclosed as oneembodiment of the invention, and one embodiment of the invention can beconstituted. The embodiment of the present invention is clear.

Embodiment 4

A battery management unit (BMU) which can be combined with a batterycell including the materials described in the above embodiment, and atransistor suitable for a circuit included in the battery managementunit will be described with reference to FIG. 22, FIGS. 23A to 23C, FIG.24, FIG. 25, FIGS. 26A to 26C, FIG. 27, and FIG. 28. In this embodiment,a battery management unit of a power storage device including batterycells that are connected in series will be particularly described.

When a plurality of battery cells connected in series are charged anddischarged repeatedly, each battery cell has different capacity (andoutput voltage depending on the capacity) from one another because ofthe variation in performance among the battery cells. The dischargecapacity of all of the battery cells connected in series depends on abattery cell with low capacity. The variation in capacities reduces thecapacity of the battery cells at the time of discharging. Charging basedon a battery cell with low capacity may cause insufficient charging.Charging based on a battery cell with high capacity may causeovercharge.

Thus, the battery management unit of the power storage device includingbattery cells connected in series has a function of reducing variationin capacities among the battery cells which causes insufficient chargingor overcharge. Although circuit structures for reducing variation incapacities among the battery cells include a resistive type, a capacitortype, and an inductor type, a circuit structure which can reducevariation in capacities among the battery cells using transistors with alow off-state current is explained here as an example.

A transistor including an oxide semiconductor in its channel formationregion (an OS transistor) is preferably used as the transistor with alow off-state current. When an OS transistor with a low off-statecurrent is used in the circuit of the battery management unit of thepower storage device, the amount of electric charge leaking from abattery can be reduced, and reduction in capacity over time can besuppressed.

As the oxide semiconductor used in the channel formation region, anIn-M-Zn oxide (M is Ga, Sn, Y, Zr, La, Ce, or Nd) is used. In the casewhere the atomic ratio of the metal elements of a target for forming anoxide semiconductor film is In:M:Zn=x₁:y₁:z₁, x₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, furtherpreferably greater than or equal to 1 and less than or equal to 6, andz₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to6, further preferably greater than or equal to 1 and less than or equalto 6. Note that when z₁/y₁ is greater than or equal to 1 and less thanor equal to 6, a CAAC-OS film as the oxide semiconductor film is easilyformed.

Here, the details of the CAAC-OS film will be described.

The CAAC-OS film is one of oxide semiconductor films having a pluralityof c-axis aligned crystal parts.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OSfilm, which is obtained using a transmission electron microscope (TEM),a plurality of crystal parts can be observed. However, in thehigh-resolution TEM image, a boundary between crystal parts, that is, agrain boundary cannot be clearly observed. Thus, in the CAAC-OS film, areduction in electron mobility due to the grain boundary is less likelyto occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in a direction substantially parallel to a samplesurface, metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology reflecting unevenness of asurface over which the CAAC-OS film is formed (hereinafter, a surfaceover which the CAAC-OS film is formed is referred to as a formationsurface) or of a top surface of the CAAC-OS film, and is arrangedparallel to the formation surface or the top surface of the CAAC-OSfilm.

On the other hand, according to the high-resolution planar TEM image ofthe CAAC-OS film observed in a direction substantially perpendicular tothe sample surface, metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2 θ) is around 31°.This peak is derived from the (009) surface of the InGaZnO₄ crystal,which indicates that crystals in the CAAC-OS film have c-axis alignment,and that the c-axes are aligned in a direction substantiallyperpendicular to the formation surface or the top surface of the CAAC-OSfilm.

Note that when the CAAC-OS film including an InGaZnO₄ crystal isanalyzed by an out-of-plane method, a peak of 2θ may also be observed ataround 36°, in addition to the peak of 2θ at around 31°. The peak of 2θat around 36° indicates that a crystal having no c-axis alignment isincluded in part of the CAAC-OS film. It is preferable that in theCAAC-OS film, a peak of 2θ appear at around 31° and a peak of 2θ notappear at around 36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially highly purifiedintrinsic” state. A highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has few carrier generationsources, and thus can have a low carrier density. Thus, a transistorincluding the oxide semiconductor film rarely has negative thresholdvoltage (is rarely normally on). The highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier traps. Accordingly, the transistor including the oxidesemiconductor film has little variation in electrical characteristicsand high reliability. Electric charge trapped by the carrier traps inthe oxide semiconductor film takes a long time to be released, and mightbehave like fixed electric charge. Thus, the transistor which includesthe oxide semiconductor film having high impurity concentration and ahigh density of defect states has unstable electrical characteristics insome cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Since the OS transistor has a wider band gap than a transistor includingsilicon in its channel formation region (a Si transistor), dielectricbreakdown is unlikely to occur when a high voltage is applied. Althougha voltage of several hundreds of volts is generated when battery cellsare connected in series, the above OS transistor is suitable for acircuit of a battery management unit which is used for such batterycells in the power storage device.

FIG. 22 is an example of a block diagram of the power storage device. Apower storage device BT00 illustrated in FIG. 22 includes a terminalpair BT01, a terminal pair BT02, a switching control circuit BT03, aswitching circuit BT04, a switching circuit BT05, a voltagetransformation control circuit BT06, a voltage transformer circuit BT07,and a battery portion BT08 including a plurality of battery cells BT09connected in series.

In the power storage device BT00 illustrated in FIG. 22, a portionincluding the terminal pair BT01, the terminal pair BT02, the switchingcontrol circuit BT03, the switching circuit BT04, the switching circuitBT05, the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07 can be referred to as a battery managementunit.

The switching control circuit BT03 controls operations of the switchingcircuits BT04 and BT05. Specifically, the switching control circuit BT03selects battery cells to be discharged (a discharge battery cell group)and battery cells to be charged (a charge battery cell group) inaccordance with voltage measured for every battery cell BT09.

Furthermore, the switching control circuit BT03 outputs a control signalSi and a control signal S2 on the basis of the selected dischargebattery cell group and the selected charge battery cell group. Thecontrol signal S1 is output to the switching circuit BT04. The controlsignal S1 controls the switching circuit BT04 so that the terminal pairBT01 and the discharge battery cell group are connected. In addition,the control signal S2 is output to the switching circuit BT05. Thecontrol signal S2 controls the switching circuit BT05 so that theterminal pair BT02 and the charge battery cell group are connected.

The switching control circuit BT03 generates the control signal S1 andthe control signal S2 on the basis of connection relation of theswitching circuit BT04, the switching circuit BT05, and the voltagetransformer circuit BT07 so that terminals having the same polarity ofthe terminal pair BT01 and the discharge battery cell group areconnected with each other, or terminals having the same polarity of theterminal pair BT02 and the charge battery cell group are connected witheach other.

An operation of the switching control circuit BT03 is described indetail.

First, the switching control circuit BT03 measures the voltage of eachof the plurality of battery cells BT09. Then, the switching controlcircuit BT03 determines that the battery cell BT09 having a voltagehigher than a predetermined threshold value is a high-voltage batterycell (high-voltage cell) and that the battery cell BT09 having a voltagelower than the predetermined threshold value is a low-voltage batterycell (low-voltage cell), for example.

As a method to determine whether a battery cell is a high-voltage cellor a low-voltage cell, any of various methods can be employed. Forexample, the switching control circuit BT03 may determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell on thebasis of the voltage of a battery cell BT09 having a highest voltage ora lowest voltage among the plurality of battery cells BT09. In thiscase, the switching control circuit BT03 can determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell bydetermining whether or not a ratio of a voltage of each battery cellBT09 to the reference voltage is the predetermined value or more. Then,the switching control circuit BT03 determines a charge battery cellgroup and a discharge battery cell group on the basis of thedetermination result.

Note that high-voltage cells and low-voltage cells are mixed in variousstates in the plurality of battery cells BT09. For example, theswitching control circuit BT03 selects a portion having the largestnumber of high-voltage cells connected in series as the dischargebattery cell group of mixed high-voltage cells and low-voltage cells.Furthermore, the switching control circuit BT03 selects a portion havingthe largest number of low-voltage cells connected in series as thecharge battery cell group. In addition, the switching control circuitBT03 may preferentially select battery cells BT09 which are nearovercharged or overdischarged as the discharge battery cell group or thecharge battery cell group.

Here, operation examples of the switching control circuit BT03 in thisembodiment will be described with reference to FIGS. 23A to 23C. FIGS.23A to 23C illustrate operation examples of the switching controlcircuit BT03. Note that FIGS. 23A to 23C each illustrate the case wherefour battery cells BT09 are connected in series as an example forconvenience of explanation.

FIG. 23A shows the case where the relation of voltages Va, Vb, Vc, andVd is Va=Vb=Vc>Vd where the voltages Va, Vb, Vc, and Vd are voltages ofa battery cell a, a battery cell b, a battery cell c, and a battery celld, respectively. That is, a series of three high-voltage cells a to cand one low-voltage cell d are connected in series. In that case, theswitching control circuit BT03 selects the series of three high-voltagecells a to c as the discharge battery cell group. In addition, theswitching control circuit BT03 selects the low-voltage cell d as thecharge battery cell group.

Next, FIG. 23B shows the case where the relation of the voltages isVc>Va=Vb>>Vd. That is, a series of two low-voltage cells a and b, onehigh-voltage cell c, and one low-voltage cell d which is close tooverdischarge are connected in series. In that case, the switchingcontrol circuit BT03 selects the high-voltage cell c as the dischargebattery cell group. Since the low-voltage cell d is close tooverdischarge, the switching control circuit BT03 preferentially selectsthe low-voltage cell d as the charge battery cell group instead of theseries of two low-voltage cells a and b.

Lastly, FIG. 23C shows the case where the relation of the voltages isVa>Vb=Vc=Vd. That is, one high-voltage cell a and a series of threelow-voltage cells b to d are connected in series. In that case, theswitching control circuit BT03 selects the high-voltage cell a as thedischarge battery cell group. In addition, the switching control circuitBT03 selects the series of three low-voltage cells b to d as the chargebattery cell group.

On the basis of the determination result shown in the examples of FIGS.23A to 23C, the switching control circuit BT03 outputs the controlsignal S1 and the control signal S2 to the switching circuit BT04 andthe switching circuit BT05, respectively. Information showing thedischarge battery cell group being the connection destination of theswitching circuit BT04 is set in the control signal S1. Informationshowing the charge battery cell group being a connection destination ofthe switching circuit BT05 is set in the control signal S2.

The above is the detailed description of the operation of the switchingcontrol circuit BT03.

The switching circuit BT04 sets the discharge battery cell groupselected by the switching control circuit BT03 as the connectiondestination of the terminal pair BT01 in response to the control signalS1 output from the switching control circuit BT03.

The terminal pair BT01 includes a pair of terminals A1 and A2. Theswitching circuit BT04 sets the connection destination of the terminalpair BT01 by connecting one of the pair of terminals A1 and A2 to apositive electrode terminal of a battery cell BT09 positioned on themost upstream side (on the high potential side) of the discharge batterycell group, and the other to a negative electrode terminal of a batterycell BT09 positioned on the most downstream side (on the low potentialside) of the discharge battery cell group. Note that the switchingcircuit BT04 can recognize the position of the discharge battery cellgroup on the basis of the information set in the control signal S1.

The switching circuit BT05 sets the charge battery cell group selectedby the switching control circuit BT03 as the connection destination ofthe terminal pair BT02 in response to the control signal S2 output fromthe switching control circuit BT03.

The terminal pair BT02 includes a pair of terminals B1 and B2. Theswitching circuit BT05 sets the connection destination of the terminalpair BT02 by connecting one of the pair of terminals B1 and B2 to apositive electrode terminal of a battery cell BT09 positioned on themost upstream side (on the high potential side) of the charge batterycell group, and the other to a negative electrode terminal of a batterycell BT09 positioned on the most downstream side (on the low potentialside) of the charge battery cell group. Note that the switching circuitBT05 can recognize the position of the charge battery cell group on thebasis of the information set in the control signal S2.

FIG. 24 and FIG. 25 are circuit diagrams showing configuration examplesof the switching circuits BT04 and BT05.

In FIG. 24, the switching circuit BT04 includes a plurality oftransistors BT10, a bus BT11, and a bus BT12. The bus BT11 is connectedto the terminal A1. The bus BT12 is connected to the terminal A2.Sources or drains of the plurality of transistors BT10 are connectedalternately to the bus BT11 and the bus BT12. Sources or drains whichare not connected to the bus BT11 and the bus BT12 of the plurality oftransistors BT10 are each connected between two adjacent battery cellsBT09.

A source or a drain which is not connected to the bus BT11 and the busBT12 of the transistor BT10 on the most upstream side of the pluralityof transistors BT10 is connected to a positive electrode terminal of abattery cell BT09 on the most upstream side of the battery portion BT08.A source or a drain which is not connected to the bus BT11 and the busBT12 of the transistor BT10 on the most downstream side of the pluralityof transistors BT10 is connected to a negative electrode terminal of abattery cell BT09 on the most downstream side of the battery portionBT08.

The switching circuit BT04 connects the discharge battery cell group tothe terminal pair BT01 by bringing one of the plurality of thetransistors BT10 which are connected to the bus BT11 and one of theplurality of transistors BT10 which are connected to the bus BT12 intoan on state in response to the control signal S1 supplied to gates ofthe plurality of transistors BT10. Accordingly, the positive electrodeterminal of the battery cell BT09 on the most upstream side of thedischarge battery cell group is connected to one of the pair ofterminals A1 and A2. In addition, the negative electrode terminal of thebattery cell BT09 on the most downstream side of the discharge batterycell group is connected to the other of the pair of terminals A1 and A2(i.e., a terminal which is not connected to the positive electrodeterminal).

An OS transistor is preferably used as the transistor BT10. Since theoff-state current of the OS transistor is low, the amount of electriccharge leaking from battery cells which do not belong to the dischargebattery cell group can be reduced, and reduction in capacity over timecan be suppressed. In addition, dielectric breakdown is unlikely tooccur in the OS transistor when a high voltage is applied. Therefore,the battery cell BT09 and the terminal pair BT01, which are connected tothe transistor BT10 in an off state, can be insulated from each othereven when an output voltage of the discharge battery cell group is high.

In FIG. 24, the switching circuit BT05 includes a plurality oftransistors BT13, a current control switch BT14, a bus BT15, and a busBT16. The bus BT15 and the bus BT16 are provided between the pluralityof transistors BT13 and the current control switch BT14. Sources ordrains of the plurality of transistors BT13 are connected alternately tothe bus BT15 and the bus BT16. Sources or drains which are not connectedto the bus BT15 and the bus BT16 of the plurality of transistors BT13are each connected between two adjacent battery cells BT09.

A source or a drain of the transistor BT13 which is not connected to thebus BT15 on the most upstream side of the plurality of transistors BT13is connected to a positive electrode terminal of the battery cell BT09on the most upstream side of the battery portion BT08. A source or adrain of the transistor BT13 which is not connected to the bus BT15 onthe most downstream side of the plurality of transistors BT13 isconnected to a negative electrode terminal of the battery cell BT09 onthe most downstream side of the battery portion BT08.

OS transistors are preferably used as the transistors BT13 like thetransistors BT10. Since the off-state current of the OS transistor islow, the amount of electric charge leaking from the battery cells whichdo not belong to the charge battery cell group can be reduced, andreduction in capacity over time can be suppressed. In addition,dielectric breakdown is unlikely to occur in the OS transistor when ahigh voltage is applied. Therefore, the battery cell BT09 and theterminal pair BT02, which are connected to the transistor BT13 in an offstate, can be insulated from each other even when a voltage for chargingthe charge battery cell group is high.

The current control switch BT14 includes a switch pair BT17 and a switchpair BT18. One end of the switch pair BT17 is connected to the terminalB1. The other ends of the switch pair BT17 extend from respectiveswitches of the switch pair BT17. One switch is connected to the busBT15, and the other switch is connected to the bus BT16. One end of theswitch pair BT18 is connected to the terminal B2. The other ends of theswitch pair BT18 extend from two switches of the switch pair BT18. Oneswitch is connected to the bus BT15, and the other switch is connectedto the bus BT16.

OS transistors are preferably used for the switches included in theswitch pair BT17 and the switch pair BT18 like the transistors BT10 andBT13.

The switching circuit BT05 connects the charge battery cell group andthe terminal pair BT02 in response to the control signal S2 bycontrolling the combination of on and off states of the transistors BT13and the current control switch BT14.

For example, the switching circuit BT05 connects the charge battery cellgroup and the terminal pair BT02 in the following manner.

The switching circuit BT05 brings a transistor BT13 connected to apositive electrode terminal of a battery cell BT09 on the most upstreamside of the charge battery cell group into an on state in response tothe control signal S2 supplied to gates of the plurality of thetransistors BT13. In addition, the switching circuit BT05 brings atransistor BT13 connected to a negative electrode terminal of a batterycell BT09 on the most downstream side of the charge battery cell groupinto an on state in response to the control signal S2 supplied to thegates of the plurality of the transistors BT13.

The polarities of voltages applied to the terminal pair BT02 can vary inaccordance with the connection structures of the discharge battery cellgroup connected to the terminal pair BT01 and the voltage transformercircuit BT07. In order to supply current in a direction for charging thecharge battery cell group, terminals with the same polarity of theterminal pair BT02 and the charge battery cell group are required to beconnected. In view of this, the current control switch BT14 iscontrolled by the control signal S2 so that the connection destinationof the switch pair BT17 and that of the switch pair BT18 are changed inaccordance with the polarities of the voltages applied to the terminalpair BT02.

The state where voltages are applied to the terminal pair BT02 so as tomake the terminal B1 a positive electrode and the terminal B2 a negativeelectrode is described as an example. Here, in the case where thebattery cell BT09 positioned on the most downstream side of the batteryportion BT08 is in the charge battery cell group, the switch pair BT17is controlled to be connected to the positive electrode terminal of thebattery cell BT09 in response to the control signal S2. That is, theswitch of the switch pair BT17 connected to the bus BT16 is turned on,and the switch of the switch pair BT17 connected to the bus BT15 isturned off. In contrast, the switch pair BT18 is controlled to beconnected to the negative electrode terminal of the battery cell BT09positioned on the most downstream side of the battery portion BT08 inresponse to the control signal S2. That is, the switch of the switchpair BT18 connected to the bus BT15 is turned on, and the switch of theswitch pair BT18 connected to the bus BT16 is turned off. In thismanner, terminals with the same polarity of the terminal pair BT02 andthe charge battery cell group are connected to each other. In addition,the current which flows from the terminal pair BT02 is controlled to besupplied in a direction so as to charge the charge battery cell group.

In addition, instead of the switching circuit BT05, the switchingcircuit BT04 may include the current control switch BT14.

FIG. 25 is a circuit diagram illustrating structure examples of theswitching circuit BT04 and the switching circuit BT05 which aredifferent from those of FIG. 24.

In FIG. 25, the switching circuit BT04 includes a plurality oftransistor pairs BT21, a bus BT24, and a bus BT25. The bus BT24 isconnected to the terminal A1. The bus BT25 is connected to the terminalA2. One ends of the transistor pair BT21 extend from a transistor BT22and a transistor BT23. Source or drain of the transistor BT22 isconnected to the bus BT24. Source or drain of the transistor BT23 isconnected to the bus BT25. In addition, the other end of the transistorpair BT21 is connected between two adjacent battery cells BT09. Theother end of the transistor pair BT21 on the most upstream side of theplurality of transistor pairs BT21 is connected to the positiveelectrode terminal of the battery cell BT09 on the most upstream side ofthe battery portion BT08. The other end of the transistor pair BT21 onthe most downstream side of the plurality of transistor pairs BT21 isconnected to the negative electrode terminal of the battery cell BT09 onthe most downstream side of the battery portion BT08.

The switching circuit BT04 switches the connection destination of thetransistor pair BT21 to one of the terminal A1 and the terminal A2 byturning on or off the transistors BT22 and BT23 in response to thecontrol signal S1. Specifically, when the transistor BT22 is turned on,the transistor BT23 is turned off, so that the connection destination ofthe transistor pair BT21 is the terminal A1. On the other hand, when thetransistor BT23 is turned on, the transistor BT22 is turned off, so thatthe connection destination of the transistor pair BT22 is the terminalA2. Which of the transistors BT22 and BT23 is turned on is determined bythe control signal S1.

Two transistor pairs BT21 are used to connect the terminal pair BT01 andthe discharge battery cell group. Specifically, the connectiondestinations of the two transistor pairs BT21 are determined on thebasis of the control signal S1, and the discharge battery cell group andthe terminal pair BT01 are connected. The connection destinations of thetwo transistor pairs BT21 are controlled by the control signal S1 sothat one of the connection destinations is the terminal A1 and the otheris the terminal A2.

The switching circuit BT05 includes a plurality of transistor pairsBT31, a bus BT34, and a bus BT35. The bus BT34 is connected to theterminal B1. The bus BT35 is connected to the terminal B2. One ends ofthe transistor pairs BT31 extend from a transistor BT32 and a transistorBT33. One end extending from the transistor BT32 is connected to the busBT34. One end extending from the transistor BT33 is connected to the busBT35. The other end of plurality of the transistor pair BT31 isconnected between two adjacent battery cells BT09. The other end of thetransistor pair BT31 on the most upstream side of the plurality oftransistor pairs BT31 is connected to the positive electrode terminal ofthe battery cell BT09 on the most upstream side of the battery portionBT08. The other end of the transistor pair BT31 on the most downstreamside of the plurality of transistor pairs BT31 is connected to thenegative electrode terminal of the battery cell BT09 on the mostdownstream side of the battery portion BT08.

The switching circuit BT05 switches the connection destination of thetransistor pair BT31 to one of the terminal B1 and the terminal B2 byturning on or off the transistors BT32 and BT33 in response to thecontrol signal S2. Specifically, when the transistor BT32 is turned on,the transistor BT33 is turned off, so that the connection destination ofthe transistor pair BT31 is the terminal B1. On the other hand, when thetransistor BT33 is turned on, the transistor BT32 is turned off, so thatthe connection destination of the transistor pair BT31 is the terminalB2. Which of the transistors BT32 and BT33 is turned on is determined bythe control signal S2.

Two transistor pairs BT31 are used to connect the terminal pair BT02 andthe charge battery cell group. Specifically, the connection destinationsof the two transistor pairs BT31 are determined on the basis of thecontrol signal S2, and the charge battery cell group and the terminalpair BT02 are connected. The connection destinations of the twotransistor pairs BT31 are controlled by the control signal S2 so thatone of the connection destinations is the terminal B1 and the other isthe terminal B2.

The connection destinations of the two transistor pairs BT31 aredetermined by the polarities of the voltages applied to the terminalpair BT02. Specifically, in the case where voltages which make theterminal B1 a positive electrode and the terminal B2 a negativeelectrode are applied to the terminal pair BT02, the transistor pairBT31 on the upstream side is controlled by the control signal S2 so thatthe transistor BT32 is turned on and the transistor BT33 is turned off.In contrast, the transistor pair BT31 on the downstream side iscontrolled by the control signal S2 so that the transistor BT33 isturned on and the transistor BT32 is turned off. In the case wherevoltages which make the terminal B1 a negative electrode and theterminal B2 a positive electrode are applied to the terminal pair BT02,the transistor pair BT31 on the upstream side is controlled by thecontrol signal S2 so that the transistor BT33 is turned on and thetransistor BT32 is turned off. In contrast, the transistor pair BT31 onthe downstream side is controlled by the control signal S2 so that thetransistor BT32 is turned on and the transistor BT33 is turned off. Inthis manner, terminals with the same polarity of the terminal pair BT02and the charge battery cell group are connected to each other. Inaddition, the current which flows from the terminal pair BT02 iscontrolled to be supplied in a direction so as to charge the batterycell group.

The voltage transformation control circuit BT06 controls operation ofthe voltage transformer circuit BT07. The voltage transformation controlcircuit BT06 generates a voltage transformation signal S3 forcontrolling the operation of the voltage transformer circuit BT07 on thebasis of the number of the battery cells BT09 included in the dischargebattery cell group and the number of the battery cells BT09 included inthe charge battery cell group and outputs the voltage transformationsignal S3 to the voltage transformer circuit BT07.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, it is necessary to prevent a charging voltage whichis too high from being applied to the charge battery cell group. Thus,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3 for controlling the voltage transformer circuitBT07 so that a discharging voltage (Vdis) is lowered within a rangewhere the charge battery cell group can be charged.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is less than or equal to that included inthe charge battery cell group, a voltage necessary for charging thecharge battery cell group needs to be secured. Therefore, the voltagetransformation control circuit BT06 outputs the voltage transformationsignal S3 for controlling the voltage transformer circuit BT07 so thatthe discharging voltage (Vdis) is raised within a range where a chargingvoltage which is too high is not applied to the charge battery cellgroup.

The voltage value of the charging voltage which is too high isdetermined in the light of product specifications and the like of thebattery cell BT09 used in the battery portion BT08. The voltage which israised or lowered by the voltage transformer circuit BT07 is applied asa charging voltage (Vcha) to the terminal pair BT02.

Here, operation examples of the voltage transformation control circuitBT06 in this embodiment are described with reference to FIGS. 26A to26C. FIGS. 26A to 26C are conceptual diagrams for explaining theoperation examples of the voltage transformation control circuits BT06for the discharge battery cell groups and the charge battery cell groupsdescribed in FIGS. 23A to 23C. FIGS. 26A to 26C each illustrate abattery management unit BT41. The battery management unit BT41 includesthe terminal pair BT01, the terminal pair BT02, the switching controlcircuit BT03, the switching circuit BT04, the switching circuit BT05,the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07.

In an example illustrated in FIG. 26A, the series of three high-voltagecells a to c and one low-voltage cell d are connected in series withreference to FIG. 21A. In that case, as described with reference to FIG.21A, the switching control circuit BT03 selects the high-voltage cells ato c as the discharge battery cell group, and selects the low-voltagecell d as the charge battery cell group. The voltage transformationcontrol circuit BT06 calculates a conversion ratio N for converting thedischarging voltage (Vdis) to the charging voltage (Vcha) on the basisof the ratio of the number of the battery cells BT09 included in thecharge battery cell group to the number of the battery cells BT09included in the discharge battery cell group.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, when a discharging voltage is applied to theterminal pair BT02 without transforming the voltage, overvoltage may beapplied to the battery cells BT09 included in the charge battery cellgroup through the terminal pair BT02. Thus, in the case of FIG. 26A, itis necessary that a charging voltage (Vcha) applied to the terminal pairBT02 be lowered than the discharging voltage. In addition, in order tocharge the charge battery cell group, it is necessary that the chargingvoltage be higher than the total voltage of the battery cells BT09included in the charge battery cell group. Thus, the transformationcontrol circuit BT06 sets the conversion ratio N larger than the ratioof the number of the battery cells BT09 included in the charge batterycell group to the number of the battery cells BT09 included in thedischarge battery cell group.

Thus, the voltage transformation control circuit BT06 preferably setsthe conversion ratio N larger than the ratio of the number of thebattery cells BT09 included in the charge battery cell group to thenumber of the battery cells BT09 included in the discharge battery cellgroup by about 1% to 10%. Here, the charging voltage is made larger thanthe voltage of the charge battery cell group, but the charging voltageis equal to the voltage of the charge battery cell group in reality.Note that the voltage transformation control circuit BT06 feeds acurrent for charging the charge battery cell group in accordance withthe conversion ratio N in order to make the voltage of the chargebattery cell group equal to the charging voltage. The value of thecurrent is set by the voltage transformation control circuit BT06.

In the example illustrated in FIG. 26A, since the number of the batterycells BT09 included in the discharge battery cell group is three and thenumber of the battery cells BT09 included in the charge battery cellgroup is one, the voltage transformation control circuit BT06 calculatesa value which is slightly larger than ⅓ as the conversion ratio N. Then,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3, which lowers the discharging voltage inaccordance with the conversion ratio N and converts the voltage into acharging voltage, to the voltage transformer circuit BT07. The voltagetransformer circuit BT07 applies the charging voltage which istransformed in response to the voltage transformation signal S3 to theterminal pair BT02. Then, the battery cells BT09 included in the chargebattery cell group are charged with the charging voltage applied to theterminal pair BT02.

In each of examples illustrated in FIGS. 26B and 26C, the conversionratio Nis calculated in a manner similar to that of FIG. 26A. In each ofthe examples illustrated in FIGS. 26B and 26C, since the number of thebattery cells BT09 included in the discharge battery cell group is lessthan or equal to the number of the battery cells BT09 included in thecharge battery cell group, the conversion ratio N is 1 or more.Therefore, in that case, the voltage transformation control circuit BT06outputs the voltage transformation signal S3 for raising the dischargingvoltage and converting the voltage into the charging voltage.

The voltage transformer circuit BT07 converts the discharging voltageapplied to the terminal pair BT01 into a charging voltage on the basisof the voltage transformation signal S3. The voltage transformer circuitBT07 applies the converted charging voltage to the terminal pair BT02.Here, the voltage transformer circuit BT07 electrically insulates theterminal pair BT01 from the terminal pair BT02. Accordingly, the voltagetransformer circuit BT07 prevents a short-circuit due to a differencebetween the absolute voltage of the negative electrode terminal of thebattery cell BT09 on the most downstream side of the discharge batterycell group and the absolute voltage of the negative electrode terminalof the battery cell BT09 on the most downstream side of the chargebattery cell group. Furthermore, the voltage transformer circuit BT07converts the discharging voltage, which is the total voltage of thedischarge battery cell group, into the charging voltage on the basis ofthe voltage transformation signal S3 as described above.

An insulated direct current (DC)-DC converter or the like can be used inthe voltage transformer circuit BT07. In that case, the voltagetransformation control circuit BT06 controls the charging voltageconverted by the voltage transformer circuit BT07 by outputting a signalfor controlling the on/off ratio (the duty ratio) of the insulated DC-DCconverter as the voltage transformation signal S3.

Examples of the insulated DC-DC converter include a flyback converter, aforward converter, a ringing choke converter (RCC), a push-pullconverter, a half-bridge converter, and a full-bridge converter, and asuitable converter is selected in accordance with the value of theintended output voltage.

The structure of the voltage transformer circuit BT07 including theinsulated DC-DC converter is illustrated in FIG. 27. An insulated DC-DCconverter BT51 includes a switch portion BT52 and a transformer BT53.The switch portion BT52 is a switch for switching the on/off state ofthe operation of the insulated DC-DC converter, and a metal oxidesemiconductor field-effect transistor (MOSFET), a bipolar transistor, orthe like is used as the switch portion BT52. The switch portion BT52periodically turns on and off the insulated DC-DC converter BT51 inaccordance with the voltage transformation signal S3 controlling theon/off ratio which is output from the voltage transformation controlcircuit BT06. The switch portion BT52 can have any of various structuresin accordance with the type of the insulated DC-DC converter which isused. The transformer BT53 converts the discharging voltage applied fromthe terminal pair BT01 into the charging voltage. In detail, thetransformer BT53 operates in conjunction with the on/off state of theswitch portion BT52 and converts the discharging voltage into thecharging voltage in accordance with the on/off ratio. As the time duringwhich the switch portion BT52 is on becomes longer in its switchingperiod, the charging voltage is increased. On the other hand, as thetime during which the switch portion BT52 is on becomes shorter in itsswitching period, the charging voltage is decreased. In the case wherethe insulated DC-DC converter is used, the terminal pair BT01 and theterminal pair BT02 can be insulated from each other inside thetransformer BT53.

A flow of operations of the power storage device BT00 in this embodimentis described with reference to FIG. 28. FIG. 28 is a flow chartillustrating the flow of the operations of the power storage deviceBT00.

First, the power storage device BT00 obtains a voltage measured for eachof the plurality of battery cells BT09 (step S001). Then, the powerstorage device BT00 determines whether or not the condition for startingthe operation of reducing variations in voltage of the plurality ofbattery cells BT09 is satisfied (step S002). An example of the startingcondition can be that the difference between the maximum value and theminimum value of the voltage measured for each of the plurality of thebattery cells BT09 is higher than or equal to the predeterminedthreshold value. In the case where the starting condition is notsatisfied (step S002: NO), the power storage device BT00 does notperform the following operation because voltages of the battery cellsBT09 are well balanced. In contrast, in the case where the condition issatisfied (step S002: YES), the power storage device BT00 performs theoperation of reducing variations in the voltage of the battery cellsBT09. In this operation, the power storage device BT00 determineswhether each battery cell BT09 is a high voltage cell or a low voltagecell on the basis of the measured voltage of each cell (step S003).Then, the power storage device BT00 determines a discharge battery cellgroup and a charge battery cell group on the basis of the determinationresult (step S004). In addition, the power storage device BT00 generatesthe control signal S1 for setting the determined discharge battery cellgroup as the connection destination of the terminal pair BT01, and thecontrol signal S2 for setting the determined charge battery cell groupas the connection determination of the terminal pair BT02 (step S005).The power storage device BT00 outputs the generated control signals S1and S2 to the switching circuit BT04 and the switching circuit BT05,respectively. Then, the switching circuit BT04 connects the terminalpair BT01 and the discharge battery cell group, and the switchingcircuit BT05 connects the terminal pair BT02 and the discharge batterycell group (step S006). The power storage device BT00 generates thevoltage transformation signal S3 based on the number of the batterycells BT09 included in the discharge battery cell group and the numberof the battery cells BT09 included in the charge battery cell group(step S007). Then, the power storage device BT00 converts thedischarging voltage applied to the terminal pair BT01 into a chargingvoltage based on the voltage transformation signal S3 and applies thecharging voltage to the terminal pair BT02 (step S008). In this way,electric charge of the discharge battery cell group is transferred tothe charge battery cell group.

Although the plurality of steps are shown in order in the flow chart ofFIG. 28, the order of performing the steps is not limited to the order.

According to the above embodiment, when an electric charge istransferred from the discharge battery cell group to the charge batterycell group, a structure where an electric charge from the dischargebattery cell group is temporarily stored, and the stored electric chargeis sent to the charge battery cell group is unnecessary, unlike in the acapacitor type circuit. Accordingly, the charge transfer efficiency perunit time can be increased. In addition, the switching circuit BT04 andthe switching circuit BT05 determine which battery cell in the dischargebattery cell group and the charge battery cell group to be connected tothe voltage transformer circuit.

Furthermore, the voltage transformer circuit BT07 converts thedischarging voltage applied to the terminal pair BT01 into the chargingvoltage based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group, and applies the chargingvoltage to the terminal pair BT02. Thus, even when any battery cell BT09is selected as the discharge battery cell group and the charge batterycell group, an electric charge can be transferred without any problems.

Furthermore, the use of OS transistors as the transistor BT10 and thetransistor BT13 can reduce the amount of electric charge leaking fromthe battery cells BT09 which do not belong to the charge battery cellgroup or the discharge battery cell group. Accordingly, a decrease incapacity of the battery cells BT09 which do not contribute to chargingor discharging can be suppressed. In addition, the variation incharacteristics of the OS transistor due to heat is smaller than that ofan S1 transistor. Accordingly, even when the temperature of the batterycells BT09 is increased, an operation such as turning on or off thetransistors in response to the control signals S1 and S2 can beperformed normally.

This embodiment can be implemented in appropriate combination with anyof the other embodiments and example.

Embodiment 5

<<Other Structure Examples of Secondary Battery>>

FIGS. 29A to 29C illustrate a secondary battery 2100 of one embodimentof the present invention. FIG. 29A is a perspective view of thesecondary battery 2100, and FIG. 29B is a top view thereof. FIG. 29C isa cross-sectional view taken along the dashed-dotted line G1-G2 in FIG.29B.

Three sides of an exterior body 2107 in the secondary battery 2100 aresealed, as illustrated in FIGS. 29A to 29C. Furthermore, the secondarybattery 2100 includes a positive electrode lead 2121, a negativeelectrode lead 2125, a positive electrode 2111, a negative electrode2115, and a separator 2103. In FIG. 29C, a positive electrode 2111, anegative electrode 2115, a separator 2103, a positive electrode lead2121, a negative electrode lead 2125, and a sealing layer 2120 areselectively illustrated for the sake of clarity. Note that some of theelectrodes may include two or more current collectors and the currentcollectors may be in contact with each other on surfaces where activematerials are not formed.

Here, some steps in the fabricating method of the secondary battery 2100illustrated in FIGS. 30A to 30D will be described with reference toFIGS. 29A to 29C.

First, the negative electrode 2115 is positioned over the separator 2103(FIG. 30A) such that a negative electrode active material layer in thenegative electrode 2115 overlaps with the separator 2103.

Then, the separator 2103 is folded such that part of the separator 2103is positioned over the negative electrode 2115. Next, the positiveelectrode 2111 is positioned over the separator 2103 (FIG. 30B) suchthat a positive electrode active material layer included in the positiveelectrode 2111 overlaps with the separator 2103 and the negativeelectrode active material layer. In the case where an electrode in whichan active material layer is formed on one surface of a current collectoris used, the positive electrode active material layer of the positiveelectrode 2111 and the negative electrode active material layer of thenegative electrode 2115 are positioned so as to face each other with theseparator 2103 therebetween.

In the case where the separator 2103 is formed using a material that canbe thermally welded, such as polypropylene, a region where the separator2103 overlap with itself is thermally welded and then another electrodeis positioned so as to overlap with the separator 2103, whereby theslippage of the electrode in the manufacturing process can be minimized.Specifically, a region which does not overlap with the negativeelectrode 2115 and the positive electrode 2111 and in which theseparator 2103 overlaps with itself, e.g., a region 2103 a in FIG. 30B,is preferably thermally welded.

By repeating the above steps, the positive electrode 2111 and thenegative electrode 2115 can overlap with each other with the separator2103 therebetween as illustrated in FIG. 30C.

Note that a plurality of negative electrodes 2115 and a plurality ofpositive electrodes 2111 may be alternately placed to be sandwichedbetween portions of the separator 2103 that is repeatedly folded inadvance.

Next, as illustrated in FIG. 30C, the plurality of positive electrodes2111 and the plurality of negative electrodes 2115 are covered with theseparator 2103.

Then, as illustrated in FIG. 30D, a region where the separator 2103overlaps with itself, e.g., a region 2103 b in FIG. 30D, is thermallywelded, whereby the plurality of positive electrodes 2111 and theplurality of negative electrodes 2115 are covered with and bundled inthe separator 2103.

Note that the plurality of positive electrodes 2111, the plurality ofnegative electrodes 2115, and the separator 2103 may be bundled using abinding material.

Since the positive electrodes 2111 and the negative electrodes 2115 arestacked through the above steps, one separator 2103 has a regionsandwiched between the plurality of positive electrodes 2111 and theplurality of negative electrodes 2115 and a region positioned so as tocover the plurality of positive electrodes 2111 and the plurality ofnegative electrodes 2115.

In other words, the separator 2103 included in the secondary battery2100 in FIGS. 29A to 29C is a single separator which is partly folded.Between the folded parts of the separator 2103, the positive electrodes2111 and the negative electrodes 2115 are sandwiched.

FIGS. 31A, 31B, 31C1, 31C2, and 31D illustrate a secondary battery 2200,which is different from the secondary battery illustrated in FIGS. 29Ato 29C. FIG. 31A is a perspective view of the secondary battery 2200,and FIG. 31B is a top view thereof. FIG. 31C1 is a cross-sectional viewof a first electrode assembly 2130, and FIG. 31C2 is a cross-sectionalview of a second electrode assembly 2131. FIG. 31D is a cross-sectionalview taken along dashed-dotted line H1-H2 in FIG. 31B. In FIG. 31D, thefirst electrode assembly 2130, the second electrode assembly 2131, andthe separator 2103 are selectively illustrated for clarity. Note thatsome of the electrodes may include two or more current collectors andthe current collectors may be in contact with each other on surfaceswhere active materials are not formed.

The secondary battery 2200 illustrated in FIGS. 31A, 31B, 31C 1, 31C2,and 31D is different from the secondary battery 2100 illustrated inFIGS. 29A to 29C in the positions of the positive electrodes 2111 a, thenegative electrodes 2115 a, and the separator 2103.

As illustrated in FIG. 31D, the secondary battery 2200 includes aplurality of first electrode assemblies 2130 and a plurality of secondelectrode assemblies 2131.

As illustrated in FIG. 31C1, in each of the first electrode assemblies2130, a positive electrode 2111 a including positive electrode activematerial layers on both surfaces of a positive electrode currentcollector, the separator 2103, a negative electrode 2115 a includingnegative electrode active material layers on both surfaces of a negativeelectrode current collector, the separator 2103, and the positiveelectrode 2111 a including the positive electrode active material layerson both surfaces of the positive electrode current collector are stackedin this order. As illustrated in FIG. 31C2, in each of the secondelectrode assemblies 2131, the negative electrode 2115 a including thenegative electrode active material layers on both surfaces of thenegative electrode current collector, the separator 2103, the positiveelectrode 2111 a including the positive electrode active material layerson both surfaces of the positive electrode current collector, theseparator 2103, and the negative electrode 2115 a including the negativeelectrode active material layers on both surfaces of the negativeelectrode current collector are stacked in this order.

As illustrated in FIG. 31D, the plurality of first electrode assemblies2130 and the plurality of second electrode assemblies 2131 are coveredwith the wound separator 2103.

Here, some steps in a fabricating method of the secondary battery 2200illustrated in FIGS. 31A, 31B, 31C1, 31C2, and 31D will be describedwith reference to FIGS. 32A to 32D.

First, the first electrode assembly 2130 is positioned over theseparator 2103 (FIG. 32A).

Then, the separator 2103 is folded such that part of the separator 2103is positioned over the first electrode assembly 2130. Next, two secondelectrode assemblies 2131 are positioned over and under the firstelectrode assembly 2130 with the separator 2103 therebetween (FIG. 32B).

Then, the separator 2103 is wound so as to cover the two secondelectrode assemblies 2131. Next, two first electrode assemblies 2130 arepositioned over and under the two second electrode assemblies 2131 withthe separator 2103 therebetween (FIG. 32C).

Then, the separator 2103 is wound so as to cover the two first electrodeassemblies 2130 (FIG. 32D).

Since the plurality of first electrode assemblies 2130 and the pluralityof second electrode assemblies 2131 are stacked through the above steps,the electrode assemblies are positioned between portions of theseparator 2103 that is spirally wound.

It is preferable that the positive electrode 2111 a of the firstelectrode assembly 2130 that is positioned on the outermost side includeno positive electrode active material layer on the outer side.

In the example illustrated in FIGS. 31C1 and 31C2, the electrodeassembly includes three electrodes and two separators; however, oneembodiment of the present invention is not limited to this example. Theelectrode assembly may include four or more electrodes and three or moreseparators. As the number of electrodes is increased, the capacity ofthe secondary battery 2200 can be further improved. Note that theelectrode assembly may include two electrodes and one separator. In thecase where the number of electrodes is small, the secondary battery 2200can have higher resistance to bending. In the example illustrated inFIG. 31D, the secondary battery 2200 includes three first electrodeassemblies 2130 and two second electrode assemblies 2131; however, oneembodiment of the present invention is not limited to this example. Thestorage battery 2200 may include more electrode assemblies. As thenumber of electrode assemblies is increased, the capacity of thesecondary battery 2200 can be further improved. The secondary battery2200 may include a smaller number of electrode assemblies. In the casewhere the number of electrode assemblies is small, the secondary battery2200 can have higher resistance to bending.

The description of FIGS. 29A to 29C can be referred to for structuresother than the positions of the positive electrodes 2111, the negativeelectrodes 2115, and the separator 2103 of the secondary battery 2200.

This embodiment can be implemented in appropriate combinations with anyof the other embodiments.

Example 1

In this example, the lithium-containing complex phosphate manufacturedaccording to one embodiment of the present invention by the methoddescribed in Embodiment 1 will be described.

First, LiOH—H₂O, FeCl₂.4H₂O, and NH₄H₂PO₄ were individually weighed sothat the molar ratio of Li:Fe:P was 2:1:1. Specifically, LiOH₂O,FeCl₂.4H₂O, and NH₄H₂PO₄ were individually weighed to be 1.6784 g,3.9758 g, and 2.3014 g, respectively.

Subsequently, LiOH₂O, FeCl_(z).4H₂O, and NH₄H₂PO₄ were individuallydissolved in 30 ml of pure water which had been subjected to nitrogenbubbling for 30 minutes, so that a solution containing Li, a solutioncontaining P, and a solution containing Fe were formed.

The solution containing Li and the solution containing P were mixedwhile being stirred in an air atmosphere, so that a mixed solution C wasformed.

Subsequently, the solution containing Fe was added dropwise little bylittle and 10 ml of pure water which had been subjected to nitrogenbubbling for 30 minutes was added to the mixed solution C while themixed solution C was stirred in an air atmosphere, so that a mixedsolution D was formed.

Subsequently, the mixed solution D was put into an autoclave includingan inner cylinder made of fluororesin and the mixed solution D washeated at 150° C. for 19 hours. During heating, the pressure inside theinner cylinder was in the range of 0.2 MPa to 0.4 MPa. After the heattreatment was performed, the inner cylinder was cooled, the syntheticmaterial in the inner cylinder was filtered, and the residue was washedwith water. For the autoclave, a mini reactor MS200-C manufactured by OMlabotech Corp. was used.

Subsequently, the obtained substance was dried in a vacuum atmosphere at60° C. for two hours, so that a synthetic material B was obtained.

Subsequently, the synthetic material B was observed with a scanningelectron microscope (SEM). FIG. 33 shows a SEM image. Two plate-likecomponents included in the lithium-containing complex phosphate, aprismatic component provided between the two plate-like components, anda space between the two plate-like components can be observed. Note thatthe SEM observation was performed using scanning electron microscopeSU8030 produced by Hitachi High-Technologies Corporation at amagnification of 30,000 times.

Subsequently, the synthetic material B was subjected to cross-sectionprocessing by focused ion beam (FIB) and the cross sections thereof wereobserved with SEM. FIGS. 34A to 34F show SEM images. Repeating thecross-section processing and the SEM observation enables obtainment ofthree-dimensional information on the component. This observation methodis called “slice and view”. FIGS. 34A to 34F are images obtained by“slice and view” and particles illustrated with the dotted lines inFIGS. 34A to 34F show the same particle. Two plate-like componentsincluded in the lithium-containing complex phosphate, a prismaticcomponent provided between the two plate-like components, and a spacebetween the two plate-like components can be observed. Note that for theFIB processing, Helios Nanolab 650 produced by FEI Company was used. Forthe SEM observation, Helios Nanolab 650 produced by FEI Company wasused.

Subsequently, X-ray diffraction (XRD) measurement of the syntheticmaterial B was performed. FIG. 35 shows XRD spectra. In FIG. 35, thehorizontal axis represents the diffraction angle 20 [deg.] and thevertical axis represents the X-ray diffraction intensity (arbitraryunit). The upper half of FIG. 35 shows the XRD spectrum of the syntheticmaterial B and the lower half shows that of LiFePO₄ (ICSD Code 92198) ofthe Inorganic Crystal Structure Database (ICSD). Since peak positionsare substantially the same, the synthetic material B can be identifiedas a crystal of lithium iron phosphate of olivine structure. Note thatfor the XRD measurement, an X-ray diffractometer D8 ADVANCE manufacturedby Bruker AXS was used and CuKα rays with a wavelength of 0.154178 nmwere used as the X-ray source.

Subsequently, particle size measurement of the synthetic material B wasperformed. FIG. 36 shows a histogram of particle size. In FIG. 36, thehorizontal axis represents the particle size [μm] and the vertical axisrepresents the proportion of particles with respect to volume [%]. Itcan be considered that small particle size distribution representsprimary particles and large particle size distribution representsaggregate particles (secondary particles). The peak of the primaryparticles was 0.68 μm. Note that for the measurement of particle size, alaser diffraction particle size analyzer SALD-2200 manufactured byShimadzu Corporation was used and measurement was performed in the rangeof 0.03 μm to 1000 μm in particle size at 51 intervals divided bylogarithmic scale.

In the above manner, the lithium-containing complex phosphate accordingto one embodiment of the present invention was synthesized and theparticle shape thereof and the like were observed in this example.

This application is based on Japanese Patent Application serial no.2016-040959 filed with Japan Patent Office on Mar. 3, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a positive electrodeactive material of a lithium-ion battery, comprising the steps of:dissolving a lithium compound, a phosphorus compound, and an M(II)compound in a solvent (M(II) is one or more of iron(II), manganese(II),cobalt(II), and nickel(II)); forming a first mixed solution containinglithium and phosphorus; forming a second mixed solution containinglithium, phosphorus, and M(II); heating the second mixed solution;obtaining a solid after heating the second mixed solution.
 2. A methodfor manufacturing a positive electrode active material of a lithium-ionbattery, comprising the steps of: dissolving a lithium compound, aphosphorus compound, and an M(II) compound in a solvent (M(II) is one ormore of iron(II), manganese(II), cobalt(II), and nickel(II)); forming afirst mixed solution containing lithium and phosphorus in an airatmosphere; forming a second mixed solution containing lithium,phosphorus, and M(II) in an air atmosphere; heating the second mixedsolution; obtaining a solid after heating the second mixed solution. 3.A method for manufacturing a positive electrode active material of alithium-ion battery, comprising the steps of: dissolving a lithiumcompound in a solvent to form a solution containing lithium; dissolvinga phosphorus compound in a solvent to form a solution containingphosphorus; dissolving an M(II) compound in a solvent to form a solutioncontaining M(II) (M(II) is one or more of iron(II), manganese(II),cobalt(II), and nickel(II)); mixing the solution containing lithium andthe solution containing phosphorus to form a first mixed solution in anair atmosphere; adding the solution containing M(II) dropwise to thefirst mixed solution while stirring the first mixed solution so as toform a second mixed solution in an air atmosphere; putting the secondmixed solution into a container resistant to heat and pressure; heatingthe second mixed solution; cooling the second mixed solution; obtaininga solid by filtration of the cooled down second mixed solution.
 4. Themethod for manufacturing a positive electrode active material of alithium-ion battery according to claim 3, further comprising the stepsof: washing the solid with water; and drying the solid.
 5. The methodfor manufacturing a positive electrode active material of a lithium-ionbattery according to claim 1, wherein the lithium compound is any one oflithium hydroxide-hydrate, lithium chloride, lithium carbonate, lithiumacetate, and lithium oxalate.
 6. The method for manufacturing a positiveelectrode active material of a lithium-ion battery according to claim 5,wherein the solvent is water.
 7. The method for manufacturing a positiveelectrode active material of a lithium-ion battery according to claim 6,wherein the phosphorus compound is a phosphoric acid or ammoniumhydrogenphosphate.
 8. The method for manufacturing a positive electrodeactive material of a lithium-ion battery according to claim 7, whereinthe second mixed solution is heated at 100° C. to 350° C. at 0.1 MPa to100 MPa for 0.5 to 24 hours.
 9. The method for manufacturing a positiveelectrode active material of a lithium-ion battery according to claim 8,wherein the solid is a lithium-containing complex phosphate having asingle-crystal grain.
 10. The method for manufacturing a positiveelectrode active material of a lithium-ion battery according to claim 1,wherein the solid comprises an H-shaped particle.
 11. A secondarybattery comprising: a positive electrode comprising the positiveelectrode active material according to claim 1; a negative electrode;and an electrolyte
 12. The method for manufacturing a positive electrodeactive material of a lithium-ion battery according to claim 2, whereinthe lithium compound is any one of lithium hydroxide-hydrate, lithiumchloride, lithium carbonate, lithium acetate, and lithium oxalate. 13.The method for manufacturing a positive electrode active material of alithium-ion battery according to claim 12, wherein the solvent is water.14. The method for manufacturing a positive electrode active material ofa lithium-ion battery according to claim 13, wherein the phosphoruscompound is a phosphoric acid or ammonium hydrogenphosphate.
 15. Themethod for manufacturing a positive electrode active material of alithium-ion battery according to claim 14, wherein the second mixedsolution is heated at 100° C. to 350° C. at 0.1 MPa to 100 MPa for 0.5to 24 hours.
 16. The method for manufacturing a positive electrodeactive material of a lithium-ion battery according to claim 15, whereinthe solid is a lithium-containing complex phosphate having asingle-crystal grain.
 17. The method for manufacturing a positiveelectrode active material of a lithium-ion battery according to claim 2,wherein the solid comprises an H-shaped particle.
 18. A secondarybattery comprising: a positive electrode comprising the positiveelectrode active material according to claim 2; a negative electrode;and an electrolyte.
 19. The method for manufacturing a positiveelectrode active material of a lithium-ion battery according to claim 3,wherein the lithium compound is any one of lithium hydroxide-hydrate,lithium chloride, lithium carbonate, lithium acetate, and lithiumoxalate, wherein the solvent is water, wherein the phosphorus compoundis a phosphoric acid or ammonium hydrogenphosphate, and wherein thesecond mixed solution is heated at 100° C. to 350° C. at 0.1 MPa to 100MPa for 0.5 to 24 hours.
 20. A secondary battery comprising: a positiveelectrode comprising the positive electrode active material according toclaim 3; a negative electrode; and an electrolyte.